SEOUL, Oct 10: South Korea said on Tuesday it could not rule out further North Korean nuclear tests given the communist state's large stockpile of weapons-grade plutonium, and ordered its military to stay on maximum alert.

Seoul, still technically at war with Pyongyang half a century after the Korean conflict, has said it will never tolerate a nuclear-armed North.

Defence Minister Yoon Kwang-Ung summoned a meeting of about 50 senior commanders, including the chairman of the Joint Chiefs of Staff and the army, navy and air force chiefs.

Mr Yoon ordered the 650,000-strong military to ‘maintain steadfast combat readiness’ to prevent North Korea miscalculating the situation.

"North Korea's nuclear test is a grave threat to stability and peace in Northeast Asia, and it is an absolutely unpardonable provocative act that defeats the expectations of our government and people," Yonhap news agency quoted him as telling the closed-door meeting.

The military chiefs assessed security and discussed the response to the announced nuclear test, the defence ministry said.

The military has increased troop numbers near land and sea borders, but is maintaining its normal official alert level.

Mr Yoon told parliament there were no immediate signs the North was preparing further nuclear tests.

But he could not rule out the possibility it might attempt to test ‘various types’ of nuclear weapons in the future, saying Pyongyang had a stockpile of up to 50 kilograms of weapons-grade plutonium.

Protesters again took to the streets to vent anger. About 70-80 demonstrators in central Seoul set alight a mock missile plastered with a North Korean flag and a picture of the North's leader Kim Jong-Il.—AFP