The US and South Korea are scrutinizing 11 underground sites across North Korea where it could conduct a third nuclear test, based on intelligence it may do so in protest of the UN Security Council sanctions, Seoul's JoongAng Ilbo newspaper reported earlier on Monday.

Tension on the Korean peninsula spiked after the North declared Saturday it would accelerate its nuclear bomb-making program by producing more plutonium and uranium, two key ingredients.

The North also threatened war with any country that tries to stop its ships on the high seas as part of new Security Council sanctions passed in response to Pyongyang's May 25 nuclear test. It conducted its first test in 2006.

South Korean President Lee Myung-bak departed for summit talks in Washington on Tuesday with President Barack Obama that are expected to be dominated by the North's nuclear and missile programs.

In Pyongyang, a massive crowd of North Koreans packed the capital's main square in a rally to condemn the U.N. resolution, footage from APTN in North Korea showed. The isolated, totalitarian regime often organizes such rallies at times of tension with the outside world.

APTN North Korea estimated the crowd at about 100,000.

'We strongly condemn and wholly reject the UN Security Council's resolution on sanctions, fabricated at the instigation of US imperialism hell-bent on its attempt at stifling' the North, Kim Ki Nam, a top Workers' Party official, told the crowd.

Participants clapped and chanted 'Condemn! Reject!' in unison, pumping clenched fists into the sky.

The North is believed to have enough weaponized plutonium for at least half a dozen atomic bombs, and a US government official said last week that Pyongyang may be preparing for another nuclear test, its third. -- AP