The United States has positioned a warship off the Korean coast as a shield against ballistic missile attack as South Korea's new president vowed swift retaliation against a North Korean strike amid soaring tensions on the peninsula, according to Reuters.

But Washington also said it had seen no worrisome mobilization of armed forces by the North Koreans despite bellicose rhetoric over a ramping up of international sanctions against Pyongyang over nuclear weapons tests.

"If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations," South Korean President Park Geun-hye told the defense minister and senior officials.

A U.S. defense official said the USS McCain, an Aegis-class guided-missile destroyer used for ballistic missile defense, was being positioned off the peninsula's southwestern coast.

"This is a prudent move that provides greater missile defense options should (they) become necessary," said the official, speaking on condition of anonymity.

The ship was not expected to participate in any exercises, the official added.