"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the United States and South Korea, as well as the declaration that the armistice that stopped the Korean War in 1953 is no longer valid.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.