SEOUL (AFP) - North Korea issued an air raid warning and put its military on alert on Thursday, South Korea's Unification Ministry said, in what appeared to be a drill at a time of heightened tensions.

The public air raid alert was issued through state radio, a ministry spokeswoman told AFP. "We assume its an air defence drill," another government official said.

Military tensions on the Korean peninsula are at their highest level for years, with North Korea - angered by United Nations sanctions imposed after its nuclear test last month - threatening a second Korean War backed by nuclear weapons.

Pyongyang has also denounced ongoing joint South Korea-United States military manoeuvres, involving nuclear-capable B-52 bombers, calling them a provocative rehearsal for invasion.

A Defence Ministry spokesman noted that North Korea had been carrying out its own military exercises and that the air raid warning was most likely part of that exercise.