Kerry: "US will defend S. Korea and itself, following N. Korea's decision to reactivate nuclear programme."

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.

North Korea has been angered by the resultant UN sanctions 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.

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.