SEOUL: Thousands of US troops converged on South Korea Friday for the start of annual joint military exercises, a report said, as tensions run high on the peninsula following North Korea's third nuclear test.

A joint air, ground and naval field training exercise known as Foal Eagle will run until April 30, involving more than 10,000 US troops along with a far greater number of South Korean personnel.

"US troops of reinforcement headed off (to South Korea) from the bases in the US, the Pacific and Japan," a military official was quoted as saying by Yonhap news agency. The exercise will involve land, air and landing operations.

AFP was unable immediately to independently confirm the start of the exercise but an earlier announcement from the defence ministry said they would begin on Friday.

Separately, US and South Korean troops will stage a computer-simulated drill named Key Resolve from March 11-21, involving 3,500 US soldiers and 10,000 South Korean troops.

Pyongyang habitually denounces the drills as a rehearsal for invasion but Seoul and Washington insist they are defensive in nature.

South Korean Defence Ministry spokesman Kim Min-Seok said Thursday that North Korea had been conducting military exercises of its own in recent weeks with greater intensity than previously.

The United States has based troops in the South since the 1950-53 Korean War and the force currently numbers 28,500.

South Korea has staged a series of drills separately or with the United States since the North launched a long-range rocket in December which it followed up with its third nuclear test last month.

North Korea said the test, which sparked international condemnation, was a direct response to UN sanctions imposed on Pyongyang after the rocket launch.

North Korea is already under international sanctions for conducting nuclear tests in 2006 and 2009.