President Barack Obama said Thursday the United States would take firm action to prevent North Korea proliferating nuclear material to other states or terror groups. "The transfer of nuclear materials or material by North Korea to states or non-state entities would be considered a grave threat to the United States and our allies," Obama said in a speech to the Australian parliament. "We would hold North Korea fully accountable for the consequences of such action." The US leader said his country would "act firmly" against any proliferation activities by North Korea and vowed that Washington's commitment to neighbouring South Korea would "never waver". The US sees Pyongyang's uranium enrichment programme as a grave threat and has repeatedly urged the communist state to take "concrete steps" towards denuclearisation before six-nation disarmament talks can resume. Officials expressed concern this week about a new domestically built light-water reactor which is soon due to start operating, saying it was a violation of UN Security Council resolutions and 2005 commitments by the North. Talks between the US, China, Japan, Russia and both Koreas opened in 2003 and resulted in a 2005 agreement, but have stalled since December 2008. The North quit the six-party talks in April 2009, a month before staging its second atomic test. It has since made numerous overtures about returning to the negotiating table but insists there be no preconditions. Washington and its allies say talks cannot resume without concrete actions including the cessation of weapons development and testing, ending enrichment activity and re-admitting UN nuclear inspectors.