US President Barack Obama said North Korea''s reported nuclear and missile tests were "a threat to international peace" and warranted "action by the international community."

"These actions, while not a surprise given its statements and actions to date, are a matter of grave concern to all nations," Obama said in a written statement.

"North Korea''s attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security."

The comments came after North Korea said it had carried out a second and more powerful nuclear test earlier Monday, despite international pressure to rein in its atomic program after years of disarmament talks.

The communist state tested an atomic bomb for the first time in October 2006. It had threatened another test after the UN Security Council censured it following a long-range rocket launch in April.

South Korea''s Yonhap news agency said the North also appears to have test-fired a short-range missile Monday from its launch site at Musudan-ri near Kilju. There was no immediate confirmation of that report.

Obama said that "the danger posed by North Korea''s threatening activities warrants action by the international community."