US warns of serious consequences after N Korean N-tests

US President Barack Obama today warned North Korea of "serious consequences" as he consulted leaders of South Korea and Japan after Pyongyang conducted its fifth nuclear test - believed to be its most powerful ever.

"The president indicated he would continue to consult our allies and partners in the days ahead to ensure provocative actions from North Korea are met with serious consequences,"

White House press secretary Josh Earnest said. He said President Obama was briefed on the situation as he flew home from a visit to Asia aboard Air Force One.

Obama consulted with South Korean President Park Geun-hye and Japan's Prime Minister Shinzo Abe in separate phone calls from the presidential aircraft.

"The president reiterated the unbreakable US commitment to the security of our allies in Asia and around the world," Earnest said.

Earlier, National Security Council spokesman Ned Price said the US was monitoring and continuing to assess the situation in view of the seismic activity on the Korean Peninsula in the vicinity of a known nuclear site.

"We are monitoring and continuing to assess the situation in close coordination with our regional partners," Price said.

His statement came after South Korean and international monitoring agencies reported an earthquake near North Korea's north-eastern nuclear test site, indicating that Pyongyang may have detonated a new atomic test to mark the 68th anniversary of the country's founding.

"Our monitoring stations picked up an unusual seismic event in the Democratic People's Republic of Korea (DPRK) today at 00:30 (UTC). So far, 25 of our stations are contributing to the analysis," CTBT Organisation's executive secretary Lassina Zerbo said in a statement.

"The event seems to have been slightly larger than the one our system recorded on 6 January this year and the location is very similar to that event. Our initial location estimate shows that the event took place in the area of the DPRK's nuclear test site.

"If confirmed as a nuclear test, this act constitutes yet another breach of the universally accepted norm against nuclear testing; a norm that has been respected by 183 countries since 1996," she said.