The US will defend itself and its treaty ally South Korea in the face of North Korean threats, US Secretary of State John Kerry has said.
In the latest measure in the wake of its third nuclear test in January, North Korea has said it will restart all facilities at its main Yongbyon nuclear complex, including a reactor mothballed in 2007.
North Korea has been angered by UN sanctions in response to its tests and by joint US-South Korea annual military drills.
Mr Kerry made the pledge while speaking at a news conference with his South Korean counterpart Yun Byung-se at the US State Department, the BBC reports.
He described recent rhetoric from the North Korean government as "unacceptable".
Mr Yun says his country stands with the US.
The US has bolstered its forces in the region after a series of threats by Pyongyang to attack US bases in the Pacific and to invade South Korea.
The Pentagon said two missile destroyers had been deployed to the region, "poised to respond to any Missile threats to our allies or our territory".
Crisis has 'gone too far'
Earlier UN Secretary-General Ban Ki-moon, a former South Korean foreign minister, said nuclear threats are not a game and the crisis had gone too far.
"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."
Chinese foreign ministry spokesman Hong Lei called for restraint from all sides to resolve the situation.