By Ju-min Park SEOUL (Reuters) - North Korea intensified its war of words against the United States on Tuesday, vowing to strengthen its nuclear deterrent after Washington warned Pyongyang of further sanctions if it did not abandon its atomic program. Last week, world leaders meeting in the United States said North Korea needed to adhere to international norms on nuclear issues and that it would face deeper isolation if it "continues down the path of provocation." The North's foreign ministry spokesman served notice via the official KCNA news agency on Tuesday that it would "bolster its nuclear deterrent as long as the United States was continuing with its hostile policies" and that it planned "countermeasures" following pressure from Washington. Under new leader Kim Jong-un, Pyongyang tried but failed to launch a long range rocket called Unha in April, breaking an agreement with the United States that would have traded food aid for access to its nuclear facilities, among other things. Many experts now believe the reclusive North is preparing for a third nuclear test, and could even use highly enriched - or weapons-grade - uranium for the first time. U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said on Tuesday that North Korea appeared to be making rapid progress in upgrading its Tonghae Satellite Launching Ground at a site also known as Musudan-ri. Satellite images "strongly suggest that this new pad is designed to launch rockets larger than the recently tested Unha, either more capable, liquid-fueled space launch vehicles or missiles with intercontinental ranges," the Washington-based institute said on its website, 38 North. Experts say North Korea already possesses enough fissile material from plutonium for at least six nuclear bombs. U.S. Special Representative for North Korea Policy Glyn Davies told reporters in Seoul on Monday that Pyongyang could expect "a swift and sure" reaction by the international community if it undertook further hostile actions. Speaking to reporters in Beijing on Tuesday after meeting Chinese officials, Davies said he had yet to study the North Korean statement. I guess I would sum it up by saying it sounds to me like more of the same. I don't know that it adds or detracts from what we already know about the North Korean point of view about what's happening at the moment," he said. China is North Korea's sole significant economic and diplomatic supporter and even it has put pressure on Pyongyang to back down on plans for a nuclear test. Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, was skeptical that the latest rhetoric from Pyongyang signaled a nuclear test was imminent. "North Korea is simply saying: 'Don't agitate or provoke us'," he said. Recent satellite imagery published by IHS Jane's Defence Weekly, a specialist defense publication, showed there had been more work at the site of earlier nuclear tests that could indicate the North was preparing for its third nuclear test. The Janes analysis showed mining carts and excavation equipment as well more debris from inside a tunnel that could be used for another test. "A third nuclear test by North Korea would be the latest move in restarting its nuclear weapons program, which it agreed to mothball in a February 29 deal with the U.S.," said Janes analyst James Hardy. Since the death of Kim Jong-il in December, Kim Jong-un, believed to be in his late 20s, has shown he will continue with his father's hardline "military first" policy. Additional reporting by Ben Blanchard in Beijing and Paul Eckert in Washington; Editing by David Chance, Mark Heinrich and Bill Trott