US and South Korean troops will stage an annual joint exercise next month to improve their combat-readiness, military authorities said Monday amid high cross-border tensions.

The 10-day Ulchi Freedom Guardian exercise will begin on August 16, the Combined Forces Command said in a statement.

Ulchi Freedom Guardian is an annual computer-assisted simulation command post exercise with a focus on "training service members while exercising senior leaders' decision-making capabilities", it said.

The exercise involves "many US soldiers stationed here as well as some 3,000 who will come from abroad", a US military spokesman told AFP, adding however that there would be no field training.

The South's defence ministry said tens of thousands of South Korean soldiers would take part in the exercise.

General James Thurman, commander of the 28,500 US troops in the South, said the exercise was designed to enhance the "inter-operability" of US and South Korean military forces and their combat-readiness.

It would present the allies "with tough and realistic training events focused on preparing, preventing and prevailing against the full range of current and future threats", he said in the statement.

Describing the exercise as defensive and routine, the statement said North Korea's army had been informed of the dates and of "the non-provocative nature of this training".

The North habitually describes such joint drills as a rehearsal for invasion.

Tensions on the peninsula have flared since the South accused the North of torpedoing one of its warships with the loss of 46 lives in March 2010.

The North denied the charge but last November shelled a border island, killing four South Koreans including two civilians.

South Korea has since then staged a series of drills alone or together with US troops as a show of strength against the North.