Barack Obama promised support for North Korea's worried neighbours
US President Barack Obama has spoken to the leaders of Japan and South Korea to assure them of US defence support after North Korea's nuclear test.
Late on Monday the UN Security Council strongly condemned Pyongyang. Diplomats have begun drafting a resolution to punish the communist state.
But North Korea appeared unbowed; South Korea said its northern neighbour could be preparing more missile tests.
North Korea announced it had carried out a nuclear test early on Monday.
In a strongly-worded statement carried by the state news agency on Tuesday, it said it was prepared to fight.
"It is clear that nothing has changed in the US hostile policy against DPRK [North Korea]... even under the new US administration," it said in an article criticising recent US moves to relocate its fighter jets.
"Our army and people are fully ready for battle... against any reckless US attempt for a pre-emptive attack," it said.
South Korea, meanwhile, announced it would become a full member of a US-led initiative aimed at controlling trafficking in weapons of mass destruction, despite warnings from Pyongyang.
Mr Obama spoke by phone late on Monday with his South Korean counterpart Lee Myung-bak and Japanese Prime Minister Taro Aso to "coordinate" reaction to North Korea's nuclear test.
Laura Trevelyan, UN correspondent
The question is what kind of sanctions against North Korea can be agreed and whether they will be effective in getting this unpredictable nation to rejoin talks on dismantling its nuclear programme.
Existing measures could be expanded - like the financial sanctions against North Korean companies involved in the nuclear programme.
Western diplomats will be watching closely to see whether China will back tough sanctions. China has been reluctant to back measures which it believes could destabilise its erratic neighbour.
The White House said they agreed to work towards "a strong United Nations Security Council resolution with concrete measures to curtail North Korea's nuclear and missile activities".
In his talks with Mr Aso, Mr Obama reiterated his country's "unequivocal commitment to the defence of Japan and to maintaining peace and security in Northeast Asia".
South Korean and Chinese defence ministers are set to discuss joint action, South Korean media reported.
The discussions followed an emergency meeting of the UN Security Council in New York, where members voiced strong opposition to the test and condemned it.
The US ambassador to the UN, Susan Rice, said America wanted "strong measures" when it starts work on the resolution.
"The US thinks that this is a grave violation of international law and a threat to regional and international peace and security," she said.
Russia's UN envoy told reporters the nuclear test was a clear violation of UN Resolution 1718. That resolution imposed sanctions on North Korea after its first test.
The UK's Ambassador to the UN, Sir John Sawers, explained that the Council had decided to act in two stages, first issuing a statement "strongly condemning and opposing what the North Koreans have done by carrying out this second nuclear test".
"And we decided to start work immediately on a further Security Council resolution in order to uphold the international peace and security in the region," he added.
More missile tests?
But there was no sign that the criticism would influence North Korea.
Oct 2006 - North Korea conducts an underground nuclear test
Feb 2007 - North Korea agrees to close its main nuclear reactor in exchange for fuel aid
June 2007 - North Korea shuts its main Yongbyon reactor
June 2008 - North Korea makes its long-awaited declaration of nuclear assets
Oct 2008 - The US removes North Korea from its list of countries which sponsor terrorism
Dec 2008 - Pyongyang slows work to dismantle its nuclear programme after a US decision to suspend energy aid
Jan 2009 - The North says it is scrapping all military and political deals with the South, accusing it of "hostile intent"
April 2009 - Pyongyang launches a rocket carrying what it says is a communications satellite
25 May 2009 - North Korea conducts a second nuclear test
South Korean military sources have warned of signs that the North may be preparing to fire anti-ship missiles off its west coast. A number of missile tests accompanied Monday's nuclear test.
The government in Seoul has announced it will delay no longer in joining the PSI - a US-led non-proliferation campaign aimed at stopping the trafficking of weapons of mass destruction by stopping and searching ships carrying suspect cargoes.
North Korea has repeatedly warned that the South's participation in the PSI would be tantamount to a declaration of war.
The heightened fears of conflict follow weeks of warnings from North Korea that it would strengthen its "deterrent", and came after the communist state walked away from long-running negotiations on its nuclear programme amid deadlock with the US.
The international community faces a difficult task, confronting what many view as a dangerously unpredictable regime, BBC world affairs correspondent David Loyn reports.
Monday's blast, which was estimated by international seismologists to have the power of a 4.5 magnitude earthquake, appears to have been much more powerful than North Korea's first nuclear test, in October 2006.
Defence officials in neighbouring Russia say it was an explosion of up to 20 kilotons, making it comparable to the American bombs that flattened Hiroshima and Nagasaki in 1945.
NORTH KOREA'S SUSPECTED NUCLEAR TEST SITE
Nuclear test on 25 May 2009 is thought to have been carried out at same site as October 2006 test