In this image taken from APTN, North Koreans are seen during a ceremony to celebrate the underground nuclear test in Pyongyang, North Korea, on Tuesday. (APTN/Associated Press)

North Korea fired two short-range missiles on Tuesday, defying calls from world leaders and the UN Security Council to stand down from its nuclear program after it tested an atomic bomb.

South Korea's state news agency, Yonhap, reported on Tuesday that ground-to-air and ground-to-ship missiles with a range of about 130 kilometres were test-fired by the North from an east coast launch pad on Tuesday.

Yonhap cited an unnamed government official as saying more missiles will be launched on Wednesday.

The firings come a day after North Korea reportedly tested an underground nuclear device believed to be a size comparable to the bombs dropped on Hiroshima and Nagasaki at the end of the Second World War.

North Korea has been under UN sanctions that bar it from nuclear and ballistic activity since its first atomic test in 2006. But Pyongyang had threatened for weeks to carry out its second reported nuclear test after the UN Security Council condemned the April 5 launch of a long-range rocket.

The North claimed the test launch was part of its development of its space program but other nations alleged it was a test of long-range missile technology.

'Clear violation'

The Security Council condemned the test on Monday as a "clear violation" of a 2006 resolution banning the regime from developing its nuclear program.

Pyongyang has been engaged in years of on-off negotiations, which have been pressing the impoverished state to give up its nuclear ambitions in return for massive aid and an end to the country's pariah status.

North Korea announced in April that it was withdrawing from the six-nation disarmament talks and said it would restore partly disabled nuclear facilities.

Monday's nuclear test prompted global condemnation with world leaders stating that North Korea was threatening world peace and international security.

Act of war

UN Secretary General Ban Ki-moon urged North Korea to engage in talks with its neighbouring countries and the United States on Tuesday.

"The only viable option at this time for North Korea to remain as a responsible member of the international community is to return to the dialogue table," Ban told reporters at a news conference in Helsinki.

Most analysts have suggested that the weapons tests were aimed at bolstering North Korean leader Kim Jong-il's grip on the country ahead of his succession. The 67-year-old allegedly suffered a stroke last year and appears to be working to secure the support from the country's military for his successor, who is expected to be his third son, some experts have said.

South Korea announced on Tuesday it will join a maritime web of more than 90 nations that intercept ships suspected of spreading weapons of mass destruction. North Korea said that the move constitutes an act of war.

The North also accused the United States of hostility on Tuesday and said its army was ready to combat the U.S. in the case of an invasion.

U.S. President Barack Obama called the test a "reckless violation of international law."

Obama said the U.S. will defend South Korea and Japan, as world leaders joined together to provide a united response to Pyongyang's nuclear belligerence.

Japan's parliament unanimously passed a resolution on Tuesday that stated the country will not "tolerate" North Korea's nuclear tests.

"Japan is considering tightening sanctions against North Korea," the resolution said.

Meanwhile, Russia suspended its Russia-North Korean intergovernmental trade and economic commission in response to the test.

With files from The Associated Press