The South Korean and US military Monday began an annual joint exercise to test defences against North Korea, a drill denounced by Pyongyang as a rehearsal for war, Bangkok Post reported.


More than 30,000 US troops, both those based in the South and some 3,000 from overseas, are taking part in the drill known as Ulchi Freedom Guardian, which ends on August 31, the US forces said in a statement.


Seoul's defence ministry could not say how many South Korean troops were taking part but Yonhap news agency put the number at 56,000.


The drill does not involve field training and is largely a computer-simulated exercise. US and South Korean forces insist it is defensive while the North has called it "a dangerous act to ignite a new war".


General James Thurman, commander of the 28,500 US troops based in the South, called Ulchi Freedom Guardian "a key exercise in strengthening the readiness of Republic of Korea (South Korean) and US forces".


On the eve of the exercise, the North's leader Kim Jong-Un visited a frontline artillery unit which carried out the deadly 2010 bombardment of a South Korean island near the disputed western sea border.


Kim praised its personnel as heroes and told them never to tolerate enemy aggression, the North's official news agency reported Saturday.


The two Koreas have remained technically at war since their 1950-53 conflict ended in an armistice, without a subsequent peace treaty.


Cross-border tensions have been high 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 went on to shell the border island in November that year, killing four South Koreans.
About 20 activists gathered outside the biggest US army base in Seoul's Yongsan district to protest at the exercise, displaying banners reading "Stop UFG (Ulchi Freedom Guardian)!" and "Sign Peace Treaty".


"This is a war game and a physical threat to the North," they said in a statement, adding that the drill heightens tensions on the peninsula.