North Korea's second most powerful leader has threatened more nuclear tests if the US maintains its "hostile" policy, Japanese media report.
Kim Yong-nam spoke of "physical steps" to deal with the US
Kim Yong-nam said the decision depended on how the US treated North Korea, Japan's Kyodo news agency said.
North Korea's foreign ministry said the country would consider any increase in US pressure to be an "act of war".
France's defence minister said North Korea's claimed nuclear test, on Monday, may have failed or was a fake.
Mr Kim said, in an interview with Kyodo, that "the issue of future nuclear tests is linked to US policy toward our country".
''If the United States continues to take a hostile attitude and apply pressure on us in various forms, we will have no choice but to take physical steps to deal with that.''
The comments were the first from a senior North Korean official since the country announced it had carried out a nuclear test on Monday.
Mr Kim also said North Korea would be willing to return to stalled six-party talks on its nuclear programme if existing sanctions were lifted.
In a statement carried by the official Korean Central News Agency, the foreign ministry also said the North was "ready for both dialogue and confrontation".
"We were compelled to prove that we have nuclear weapons to prevent the increasing threat of war by the US and protect our sovereignty and survival," the statement said.
French Defence Minister Michele Alliot-Marie said in a radio interview that an "explosion of limited size" had been detected on Monday when North Korea carried out the claimed test.
N KOREA NUCLEAR PROGRAMME
Believed to have 'handful' of nuclear weapons
But not thought to have any small enough to put in a missile
Could try dropping from plane, though world watching closely
"Given its weak power, it is hard to say if it was a very large, but traditional, type of explosion or else a nuclear explosion... If it was a nuclear explosion, it was a failed explosion," she said.
The comments from Pyongyang came after Australian Foreign Minister Alexander Downer said that "information from contacts" suggested further testing may be imminent.
"We have very real concerns that they may conduct another nuclear test and that they may do so very soon," he said.
The UN Security Council is debating what multilateral sanctions North Korea is to face in response to the claimed nuclear test.
But Japan is to impose its own restrictions, which are due to be discussed at a meeting of the country's security council on Wednesday, Japanese media report.
Likely measures are said to include barring North Korean ships from Japanese port, a ban on trade between the two countries and tighter curbs on North Koreans wishing to visit Japan.
The UN Security Council is due on Wednesday to continue discussing a draft resolution of punitive sanctions proposed by the US.
The US proposal includes halting trade in material that could be used to make weapons of mass destruction; inspections of cargo going in and out of North Korea; a ban on imports of luxury goods and a ban on financial transactions used to support nuclear proliferation.
There is agreement in the Security Council that North Korea should face punitive measures.
US SANCTIONS PROPOSAL
Halting trade in material that could be used to make weapons of mass destruction
Inspections of cargo going in and out of North Korea
The ending of financial transactions used to support nuclear proliferation
A ban on the import of luxury goods
The US wants the sanctions to be brought under Chapter Seven of the UN Charter, which means they would be mandatory and ultimately enforceable by military means.
But China, Russia and South Korea have expressed varying degrees of opposition to such a resolution.
BBC correspondent Laura Trevelyan, who is at the UN headquarters in New York, says diplomats hope to reach agreement by the end of the week.
The underground test reportedly took place in Gilju in Hamgyong province at 1036 (0136 GMT) on Monday morning.
Russia is the only country to have confirmed that it was a nuclear explosion.
Seoul is borrowing a sophisticated radioactivity detector from Sweden to help determine whether a nuclear test was carried out, the South Korean science and technology ministry told Yonhap news agency.