Story highlights North Korea is verbally "firing back" at recent moves by the U.S., an analyst says 
 
 Pyongyang has expressed anger that the U.S. is flying B-52s over South Korea 
 
 The flights are part of annual military exercises 
 
 Tensions on the Korean Peninsula have flared since the North's latest nuclear test 
 
 The North Korean military issued a fresh burst of ominous rhetoric Thursday, warning that U.S. bases in Guam and Japan are within its "striking range." 
 
 The statement from the Supreme Command of the Korean People's Army, carried by the North's state-run news agency, follows the announcement by the United States this week that its B-52 bombers were making flights over South Korea as part of annual military exercises. 
 
 "The U.S. should not forget that the Andersen Air Force Base on Guam where the B-52s take off and naval bases in Japan proper and Okinawa where nuclear-powered submarines are launched are within the striking range of the DPRK's precision strike means," the North Korean military said Thursday. 
 
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 Despite Pyongyang's rhetoric, no U.S. Navy submarines are based in Okinawa or anywhere else in Japan, although they may make calls at U.S. bases there. DPRK is short for Democratic People's Republic of Korea, North Korea's official name. 
 
 Citing what it called the United States' "nuclear blackmail and threat," the North said it that it, too, would "take corresponding military actions." 
 
 Pyongyang had already reacted angrily to the B-52 flights, warning Wednesday of "strong military counteraction" if the planes made more sorties over the Korean Peninsula. 
 
 Angry words after new sanctions 
 
 Tensions have spiked in the region since North Korea carried out its latest underground nuclear test last month, the first under its new young leader Kim Jong Un, prompting the United Nations Security Council to respond by toughening sanctions on the secretive regime. 
 
 The sanctions enraged the North further, and during the week when the Security Council was voting on them, it ratcheted up its threats, suggesting it could carry out a pre-emptive nuclear strike on the United States and South Korea. 
 
 Although analysts and American officials say Pyongyang is still a long way from being able to carry out such an attack on the U.S. mainland, the United States' bases in Japan and Guam appear to be within range of the North's conventional weapons. 
 
 In its comments Thursday, "the Kim Jong Un regime is just firing back" at its enemies in response to the B-52 flights and the military exercises of which they're a part, said Lee Jung-hoon, a professor of international relations at Yonsei University in Seoul. 
 
 He said that the U.N. sanctions were also likely to be fueling the North's ire, but that the regime's threats were largely driven by domestic concerns. 
 
 "They're doing all this to prop up the regime," Lee said. 
 
 The military first strategy 
 
 Since Kim took over from his father as the head of the North Korean government in late 2011, he appears to have maintained the pursuit of a stronger military deterrent rather than adopting a more conciliatory approach to relations with South Korea and the United States. 
 
 The result has been a major foreign policy headache for U.S. President Barack Obama, with Kim showing more interest in dialogue with the basketball star Dennis Rodman than U.S. diplomats. 
 
 As well as the nuclear test, his government has conducted two long-range rocket launches -- one that failed and one that succeeded -- both of which were widely viewed as tests of ballistic missile technology. 
 
 Kim has also revamped the military's leadership, ousting some generals and promoting others. 
 
 The nuclear test and rocket launches have shattered hopes of any new talks on the North's nuclear program in the near future. 
 
 Indeed, Pyongyang has said recently that the program is not up for negotiation and announced that previous nonaggression treaties with Seoul no longer apply. 
 
 A new North Korea 
 
 The regime's unabashed declarations about its nuclear weapons, which it had been much cagier about in the past, show that "it's not the same North Korea as 10 years ago," Lee said. 
 
 The United States and other global powers need to "formulate a whole new strategy" to address the changed situation, according to Lee. 
 
 "The way we've been dealing with it for the past two decades has failed," he said of the regime. "It has become close to being able to deploy nuclear weapons." 
 
 Although U.S. officials say they don't believe North Korea is in a position to strike the United States at the moment, Defense Secretary Chuck Hagel last week announced plans to deploy additional ground-based missile interceptors on the West Coast as part of efforts to enhance the nation's ability to defend itself from attack. 
 
 "The reason that we are doing what we are doing, and the reason we are advancing our program here for homeland security, is to not take any chances, is to stay ahead of the threat and to assure any contingency," Hagel said at the time. ||||| US defence secretary issues warning after Pyongyang threatens to launch missiles into the sea near US Pacific island territory 
 
 James Mattis, the US defence secretary, has warned that a North Korean missile attack aimed at US territory “could escalate into war very quickly”, saying US forces would know “within moments” if one was heading towards Guam, home to military bases and 160,000 people. 
 
 If a missile was judged to be about to hit Guam, Mattis said, “we will take it out” – a presumed reference to US missile defence systems around the island. If North Korean missiles are headed towards the seas around Guam, the defence secretary said it would be up to the president to decide how to respond. 
 
 North Korea's Kim to assess 'foolish Yankees' before deciding on Guam missile attack Read more 
 
 Kim Jong-un appeared on Tuesday to signal a pause in the escalating war of words with Donald Trump, saying he was prepared to watch US actions in the region “a little more” before ordering a planned launch. 
 
 The North Korean military had threatened to fire four intermediate range missiles at the waters around Guam as a warning shot against the US, if it persists in flying its heavy bombers, based on the Pacific islands, over South Korea. The last reported sortie by B-1B bombers over the Korean peninsula was on 7 August 7. 
 
 In 1939, I didn’t hear war coming. Now its thundering approach can’t be ignored | Harry Leslie Smith Read more 
 
 In a statement issued on Monday, North Korean state media warned that joint exercises by US and South Korea, due to start on 21 August, could trigger an accidental war at a time of high tensions. 
 
 “The US should think twice about the consequences,” the statement said. “We are watching every move of the US.” 
 
 US military officials were quoted on Monday as saying that intermediate range North Korean missiles were observed to have been moved recently, but it was not clear whether the movements were in preparation for a launch aimed at Guam. 
 
 Last week Donald Trump warned Pyongyang that the US would respond with “fire and fury” to any further threats, a warning the North Korean regime almost immediately defied with its missile threat against Guam. The US president escalated the rhetoric once more by declaring US military options were “locked and loaded” and that the North Korean ruler, Kim Jong-un, would “truly regret” any attack on Guam or other US or allied territory. 
 
 Quick guide Are US defences strong enough to ward off North Korean missiles? Show Hide What kind of anti-missile defences does the US possess? The US has various anti-missile options, some designed to take down missiles at short-range and others for medium-to-long-range. The US relies heavily on the US Patriot missile and the Terminal High-Altitude Area Defence (THAAD). The US deployed THAAD to South Korea this year to defend against medium-range missiles. There is a three-phased defence system: ground-based missiles on the Korean peninsula; US naval ships stationed in the Pacific; and two bases in Alaska and California that can launch an estimated 36 interceptors. 
 
 Is the US system robust enough to stop a North Korean missile attack? No air defence system offers anything like a complete guarantee of success. The Pentagon offer repeated assurances that air defence systems would be more than a match for any North Korean attack. But when missile defence systems have been put to the test over the last few decades, the performance has been far from reassuring. 
 
 The US provided anti-missile defence systems to Israel and Saudi Arabia during the First Gulf War as protection against Iraq's Scud missiles. It was initially claimed that they had shot down 41 of 42 missiles fired by Iraq. But eventually it was acknowledged that only a few missiles had been hit. 
 
 Recent tests of interceptors have provided little comfort – with success rates of around 50% on average. The Pentagon celebrated in May when it destroyed a mock warhead over the Pacific but overall the performance has been spotty. Since the newest intercept system was introduced in 2004 only four of nine intercept attempts have been successful. Of the five tests since 2010, only two have been successful. 
 
 On Monday, James Mattis and Rex Tillerson, the secretaries of defence and state, sought to clarify the US position. In a commentary in the Wall Street Journal that made no reference to Trump’s heated rhetoric, but claimed his administration was following a different policy from its predecessor. 
 
 “We are replacing the failed policy of ‘strategic patience’, which expedited the North Korean threat, with a new policy of strategic accountability,” Mattis and Tillerson wrote. 
 
 They argued the “new” policy was based on a combination of diplomatic and economic pressure on Pyongyang, coupled with the deployment of missile defence in the region and an overwhelming deterrent to any North Korean attack. They did not make clear how the policy was in any way different from the Obama administration’s approach, which comprised the same elements. 
 
 
 
 Guam: life in the nuclear firing line – in pictures Read more 
 
 In a visit to Seoul on Monday, the chairman of the US joint chiefs of staff, Gen Joseph Dunford, said Washington was relying primarily on economic and diplomatic means of dealing with North Korea. 
 
 “The United States military’s priority is to support our government’s efforts to achieve the denuclearisation of the Korean peninsula through diplomatic and economic pressure,” Dunford said, according to a South Korean government statement. “We are preparing a military option in case such efforts fail.” 
 
 Meanwhile a new study claimed that North Korea probably acquired the rocket engines for the two intercontinental ballistic missiles it tested in July from Ukrainian or Russian sources within the past two years, 
 
 
 
 A report published on Monday by the International Institute for Strategic Studies said that the ICBMs used a rocket engine of a type that marked a radical departure from those the North Koreans had been experimenting with before and which strongly resemble a Soviet-era design. 
 
 “An unknown number of these engines were probably acquired though illicit channels operating in Russia and/or Ukraine,” the IISS report said. 
 
 The two missile launches last month raised tensions with Washington because they showed that Pyongyang had mastered multistage ICBM technology that would ultimately allow it to hit the US mainland. 
 
 Experts differ on whether the two missiles tested on 4 and 28 July would have been able to carry a payload as heavy as a nuclear warhead far enough to strike the US west coast. However, there is a growing body of opinion that North Korea has mastered the science of making warheads small enough to put on a missile and constructing a vehicle capable of shielding a warhead through the heat of re-entering the Earth’s atmosphere. 
 
 Q&A Why is North Korea threatening Guam? Show Hide Guam, a 210 sq mile sovereign US territory in the western Pacific Ocean, is used by America as a strategic military base. Almost a third of its land is controlled by the US military and about 6,000 American troops are based there. 
 
 The island's location, within range of North Korean medium- and long-range missiles, and military significance to the US make it a logical target for Pyongyang. 
 
 As recently as Monday, two US air force B-1B bombers flew from Guam to join their counterparts from South Korea and Japan for a mission over the Korean peninsula, about 2,100 miles away. Read more 
 
 
 
 The IISS report by the missile expert Michael Elleman said the Soviet-era engine – the RD-250 – that may have been used in the North Korean ICBM would had been modified from twin combustion chamber to single chamber to meet Pyongyang’s military needs and in a way that would have required “experts with a rich history of working with the RD-250”. 
 
 “Such expertise is available at Russia’s Energomash concern and Ukraine’s KB Yuzhnoye. One has to conclude that the modified engines were made in those factories,” Elleman said. 
 
 Japan fears the once distant threat of North Korean missiles is becoming real | Justin McCurry Read more 
 
 Yuzhnoye is the design arm of a larger Ukrainian weapons producer, Yuzhmash. It has experienced rapid contraction since the fall of a pro-Moscow government in Kiev in 2014 and the start of a conflict with Russia over eastern Ukraine. 
 
 A previous attempt to steal secrets from Yuzhnoye was described in a UN report on illegal North Korean weapons procurement. In 2011, the Ukrainian authorities arrested two North Korean agents who it said had approached a Yuzhnoye employee to sell them photographs of missile research including engine design. The employee informed the Kiev authorities who set up a sting operation to catch the two spies. 
 
 
 
 On Monday, the Kiev government denied the technology used in the North Korean missiles had come from the company. 
 
 “This information is not based on any grounds, provocative by its content, and most likely provoked by Russian secret services to cover their own crimes,” Oleksandr Turchynov, the secretary of the Ukrainian national security and defence council, said in a statement. 
 
 
 
 Other US experts have questioned whether the evidence is strong enough to prove where the technology used in the North Korean ICBMs came from. 
 
 Xu Tianran, a researcher on North Korea and missiles in Beijing, said it was very likely that the North Korean rocket engines were based on the Soviet designs, but argued the available pictures were too blurry to say for sure it was a RD-250. As for the modification, Xu said: “I would bet it is produced locally.”
– After the US announced it's flying B-52 bombers over South Korea, the North is issuing its latest warning: US military bases in Guam and Japan are in its "striking range." Said the country's military: "The US should not forget that the Andersen Air Force Base on Guam where the B-52s take off and naval bases in Japan ... where nuclear-powered submarines are launched are within the striking range of (North Korea's) precision strike means." Pyongyang said it would "take corresponding military actions" over US "nuclear blackmail and threat." The bases do seem to be within the North's conventional-weapons range, CNN notes. Meanwhile, the North announced an air raid warning today, calling on military units to be ready, according to South Korea—but the warning was apparently linked to a defense drill, Reuters reports.