Story highlights South Korean workers enter industrial complex despite severing of hotline

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

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 wouldn't be smart to dismiss its provocative behavior as pure bluster," that official said.

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"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 wouldn'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.

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 can'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.

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.

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.

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.