The United States is committed to defending close ally South Korea, its top military officer said, amid another spike in tensions between Seoul and North Korea.

General Martin Dempsey made the comments in a phone call to a senior South Korean counterpart on Friday "and reiterated the strength of the US-ROK (South Korea) alliance," a Pentagon statement said, as a North Korean deadline loomed for Seoul to halt loudspeaker propaganda broadcasts across the border or face military action.