Yeonpyeong Island , South Korea ( CNN ) -- The Obama administration on Wednesday slammed North Korea 's pugnacious rants toward South Korea and the West and a U.S. intelligence official called the strident remarks worrisome . `` The ratcheting up of rhetoric is of concern to us , '' the official said . The question is whether this is `` just rhetoric , '' he said . Or , `` are things happening behind the scenes indicating the blustering has something to it . '' Another U.S. official said there is a lot of uncertainty about North Korea 's intentions . `` North Korea is not a paper tiger so it would n't be smart to dismiss its provocative behavior as pure bluster , '' that official said . U.S. says it sent B-2 stealth bombers over South Korea . `` What 's not clear right now is how much risk ( North Korean leader ) Kim Jong Un is willing to run , to show the world and domestic elites that he 's a tough guy . His inexperience is certain -- his wisdom is still very much in question . '' North Korea earlier said it was cutting off a key military hotline with South Korea amid high tensions between the two sides . `` Under the situation where a war may break out any moment , there is no need to keep north-south military communications , '' the head of a North Korean delegation told the South by telephone Wednesday , according to the North 's state-run Korean Central News Agency . There are several hotlines between North and South Korea . Earlier this month , Pyongyang disconnected a Red Cross hotline that ran through the border village of Panmunjom and was used by officials on both sides , according to the South Korean Unification Ministry . And senior U.S. officials do not believe the cutting off of some communication by North Korea in itself is indicative of more dramatic action or is conclusive . The officials note that North Korea has cut these links before , some of them multiple times . `` It 's part of the current threat-of-the-day pattern . I would n't extrapolate it to anything more conclusive , '' one official said . `` They want attention and they want to scare people both inside and outside their country . '' Officials see steps such as cutting off communication are more substitutes for doing other more dangerous things rather than precursors to more dangerous things . `` That is certainly our fervent hope , '' the official said . Analysis : what 's Kim Jong Un up to ? At the same time , there is concern about a North Korean miscalculation during this time . The officials said the lack of communication could complicate and hamper the ability of all nations involved ( including North Korea , South Korea and the United States ) to control and moderate any action -- and cycle of reaction should one begin as the result of a North Korean miscalculation . The North linked its move to annual joint military exercises by South Korea and the United States , which it has cited in a string of threats against the two countries in recent weeks . Tougher sanctions approved by the U.N. Security Council also may have fueled its anger . `` It is important the U.S. send a message . In terms of the military side , the U.S. has clearly sent a message , '' the intelligence official said . `` When people engage in this sort of rhetoric , you ca n't appear as if you are not responding , '' the official said . The intelligence community has been providing the Obama administration with assessments of Kim Jung Un 's control of the regime , but the official would not provide any details of that assessment . Administration officials also regretted the tough talk from Pyongyang . State Department spokesman Patrick Ventrell cited `` more bellicose rhetoric and threats ( that ) follow a pattern designed to raise tensions and intimidate others . '' Josh Earnest , White House principal deputy press secretary , said the United States is committed to ensuring the security of its allies , such as South Korea . `` The North Koreans are not going to achieve anything through these threats and provocations . They 're only going to further isolate the North Koreans and undermine international efforts to bring peace and stability to northeast Asia , '' Earnest said . Pentagon spokesman George Little spoke on the government 's nuclear threats to the United States and `` its more achievable threats to attack South Korean military units and shell border islands . '' `` We take their rhetoric seriously , whether it 's outside the norm which it sometimes is , or seems to suggest a more direct threat . And if you look at what they 've said recently , it 's been extremely provocative , threatening and bellicose . And it 's a complete mystery to me why they would deem it in their own interest to launch this type of rhetoric at us and our allies , '' Little said . Korean nightmare : Experts ponder potential conflict . The North 's announcement Wednesday appeared likely to affect the movement of people in and out of the Kaesong Industrial Complex , a joint economic cooperation zone between the two Koreas situated on the North 's side of the border . `` The measure taken by North Korea is not beneficial for the stable operation of the Kaesong Industrial Complex , and we urge them to withdraw the measure , '' the Unification Ministry said . On Thursday morning , the day after the North said it was severing the line , South Korean workers were able to cross the border and enter the industrial zone , the semiofficial South Korean news agency Yonhap reported , citing the Immigration and Quarantine office in Paju , near the border . An initial group of 197 workers went over the border at 8:30 a.m. local time ( 7:30 p.m. Wednesday ET ) after North Korea gave the regular approval for their movement by phone through the industrial district 's management committee , Yonhap said . A total of 530 South Koreans were due to enter the Kaesong complex on Thursday , and 511 are scheduled to come back into South Korea , according to Yonhap . North Korea 's threats : Five things to know . A symbol of North-South cooperation , the Kaesong complex is also seen as an important source of hard currency for the regime in Pyongyang . The North previously cut off the Kaesong military hotline in March 2009 -- also during annual U.S.-South Korean military exercises -- but later reinstated it , according to Yonhap . The slew of recent fiery rhetoric from Pyongyang has included threats of pre-emptive nuclear strikes against the United States and South Korea , as well as the declaration that the armistice that stopped the Korean War in 1953 is no longer valid . Behind the veil : A rare look at life in North Korea . On Tuesday , the North said it planned to place military units tasked with targeting U.S. bases under combat-ready status . Most observers say North Korea is still years away from having the technology to deliver a nuclear warhead on a missile , but it does have plenty of conventional military firepower , including medium-range ballistic missiles that can carry high explosives for hundreds of miles . The heightened tensions on the Korean Peninsula came after the North carried out a long-range rocket launch in December and an underground nuclear test last month , prompting the U.N. Security Council to step up sanctions on the secretive regime . Island faces North Korean threat up close . CNN 's K.J . Kwon reported from Yeonpyeong Island , CNN 's Pam Benson and Elise Labott reported from Washington.Jethro Mullen reported and wrote from Hong Kong . CNN 's Joe Sterling contributed to this report .
NEW : South Korean workers enter industrial complex despite severing of hotline . NEW : The North clears them to cross the border through another line , Yonhap says . U.S. intelligence official expresses concern about `` ratcheting up of rhetoric '' The announcement follows a slew of recent threats from North Korea .