The US will defend itself and its “treaty ally” South Korea in the face of North Korean threats, Secretary of State John Kerry has said.
Mr Kerry made the pledge while speaking at a news conference with his South Korean counterpart Yun Byung-se.
Pyongyang has taken a series of defiant measures in the wake of its third nuclear test in February.
On Wednesday, the North delayed the opening of the jointly-run Kaesong industrial zone, the South said.
Pyongyang has previously threatened to close the complex as part of its standoff with Washington and Seoul.
North Korea has been angered by UN sanctions imposed after its nuclear test and joint US-South Korea annual military drills.
Mr Kerry described recent rhetoric from the North Korean government as “unacceptable”.
The Pentagon said two missile destroyers, the USS Decatur and the USS McCain, had been deployed to the region.
“They will be poised to respond to any missile threats to our allies or our territory,” Pentagon spokesman George Little said.
Tensions on the Korean Peninsula are at their highest for years
On Tuesday Pyongyang announced plans to restart its main Yongbyon nuclear complex, which Mr Kerry said would be a “provocative and serious” move.
Earlier, UN Secretary General Ban Ki-moon said the crisis had “gone too far” and called for urgent talks with the North.
“Things must begin to calm down, there is no need for the DPRK [North Korea] to be on a collision course with the international community. Nuclear threats are not a game,” Mr Ban said.
The US has urged China and Russia to apply pressure on Pyongyang to change its Yongbyon plans.
Chinese foreign ministry spokesman Hong Lei called for restraint from all sides to resolve the “complex and sensitive” situation.
Russia’s foreign ministry, meanwhile, warned against escalating tensions on the Korean peninsula.
“The main objective is to avoid the forceful military scenario. It is not acceptable to use the situation there (on the Korean peninsula) to try to reach someone’s specific military and political aims,” said ministry spokesman Grigoriy Logvinov.
The BBC’s Lucy Williamson in Seoul says Yongbyon has provided the North with successful leverage in the past.
The complex offers Pyongyang two ways of making nuclear bombs – a uranium enrichment facility, and a nuclear reactor, from which the spent fuel can be turned into plutonium.
Restarting it gives the regime several cards to play: it secures more material for its nuclear programme, and also re-focuses world attention on how to stop it, our correspondent says.
The reactor at Yongbyon – which was the source for plutonium for North Korea’s nuclear weapons programme – was closed in July 2007 as part of a disarmament-for-aid deal.
The cooling tower at the facility was later destroyed, but then the disarmament deal stalled.
Part of the reason the agreement fell apart was because the US did not believe Pyongyang was fully disclosing all of its nuclear facilities – a suspicion later bolstered when North Korea unveiled a uranium enrichment facility at Yongbyon to US scientist Siegfried Hecker in 2010.
While it appeared to be for electricity generation purposes, Mr Hecker said the facility could be readily converted to produce highly-enriched uranium for bombs.
In a November 2010 report following his visit to Yongbyon, Mr Hecker said that based on what he saw, he believed North Korea could “resume all plutonium operations within approximately six months” at Yongbyon if so inclined.
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