North Korea is facing further international criticism after it reportedly fired two short-range missiles off its east coast.
It comes after the hermit state tested its second nuclear device and fired off three short-range, surface-to-air missiles from an east coast missile base at the weekend, earning it international condemnation.
And in a move certain to compound tensions in the region, South Korea said it would join a US-led initiative to intercept ships suspected of carrying weapons of mass destruction, which Pyongyang has warned it would consider a declaration of war.
A government source in Seoul said the North had test-fired one surface-to-air and one surface-to-ship missile off its east coast. The missiles had a range of about 80 miles.
North Korea could also launch more short-range missiles, perhaps toward a disputed sea border with the South, South Korean government sources said.
The UN Security Council has unanimously condemned North Korea's nuclear test, saying it was a "clear violation" of a resolution passed in 2006 after Pyongyang's first atomic test.
A statement from the council said: "The members of the Security Council have decided to start work immediately on a Security Council resolution on this matter."
North Korea said the trial was more powerful than the previous test in 2006 and was "aimed at strengthening its self-defence nuclear deterrent in every way".
President Barack Obama was also critical, saying: "These actions, while not a surprise given its statements and actions to date, are a matter of grave concern to all nations."
He added: "North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security."
Prime Minister Gordon Brown condemned the nuclear and missile tests as "erroneous, misguided and a danger to the world".
He said in a statement: "This act will undermine prospects for peace on the Korean peninsula and will do nothing for North Korea's security. The international community will treat North Korea as a partner if it behaves responsibly. If it does not then it can expect only renewed isolation."
South Korea, China and Japan also spoke out against the test, maintaining it was unacceptable.
In 2007, North Korea agreed to abandon all its nuclear programmes in six-party talks with South Korea, China, Japan,
Russia and the US.
It promised to shutdown the Yongbyon nuclear plant as part of an aid-for-disarmament deal and in response, the US removed North Korea from its
terrorism blacklist.
But the US suspended energy aid in December 2008 following a breakdown in international talks and Pyongyang's failure to verify the shutdown of its facility.
In April, North Korea launched a rocket over northern Japan, which was widely considered to be a test of long-range missile
technology.
This prompted Tokyo to extend additional
economic sanctions on the communist state and to deploy missile interceptors to the area.
In the wake of last month's rocket launch, Japan had called for a fresh resolution by the UN Security Council to declare Pyongyang in violation of a previous resolution banning the firing of of ballistic missiles.