The North's KCNA news agency, renowned for blustering anti-U.S. rhetoric, said pressure from Washington to rein in its nuclear program would be tantamount to a declaration of war.
Japan's Chief Cabinet Secretary Yasuhisa Shiozaki said new sanctions, adding to measures imposed after Pyongyang test-fired missiles in July, included barring all North Korean ships from Japanese ports and banning imports from the country. The decision, agreed by the National Security Council, would be formally approved by cabinet on Friday.
Tokyo has also been pressing for U.N. sanctions against Pyongyang, and the Security Council is likely to make a decision by Friday. A broad consensus appears to be building but some reservations have been expressed by China and Russia, North Korea's largest trading partners.
In the North Korean capital, the country's No. 2 leader said whether Pyongyang conducted more tests depended on Washington.
"The issue of future nuclear tests is linked to U.S. policy toward our country," Kyodo news agency quoted Kim Yong-nam as saying in a meeting with a Kyodo delegation.
"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," he added.
Pyongyang's KCNA news agency was characteristically dramatic in its portrayal of the situation.
"If the U.S. increases pressure upon the DPRK (North Korea), persistently doing harm to it, it will continue to take physical countermeasures, considering it as a declaration of a war," it quoted a Foreign Ministry spokesman as saying. But it added that North Korea was ready for both dialogue and confrontation.
Ignoring U.N. warnings, North Korea announced on Monday that it had conducted its first-ever nuclear test. It said a U.S. "threat of nuclear war and sanctions" forced its hand.
There were fears, later discounted, that Pyongyang might conduct another test on Wednesday. NHK started the day saying that Tokyo was checking reports of a second blast in North Korea, but the reports were never substantiated.
Experts have been unable so far to confirm that the earth tremor detected on Monday was indeed a nuclear test. U.S. envoy to Japan Thomas Schieffer said they might never be able to do so.
"Because the yield was so small, we may not be able to make a real determination at all," Schieffer told Japanese executives.
In September, Japan approved new financial sanctions effectively freezing remittances and the transfer of funds from Japan by groups suspected of having links to Pyongyang programs for developing missiles and weapons of mass destruction.
But economists said the effect on North Korea of banning trade would be more symbolic than real unless Pyongyang's key trading partners such as China and Russia joined in.
China and Russia, which both border North Korea, met other veto-holding members of the U.N. Security Council on Tuesday to discuss a range of sanctions proposed by the United States and Japan to pressure Pyongyang to abandon its nuclear program.
"I think that there has to be some punitive actions," said Beijing's U.N. ambassador, Wang Guangya. "We need to have a firm, constructive, appropriate but prudent response to North Korea's nuclear threat."
Russia called Monday's reported test a "colossal blow" to the non-proliferation regime but, like China, insisted an eventual U.N. resolution not leave the way open for the use of force.
The United States, France and Britain, the three other permanent Council members, agreed that tough measures were needed fast. Diplomats hoped a resolution could be adopted by Friday on an array of weapons-related and financial sanctions.
Human Rights Watch urged that emergency food aid to the impoverished North be continued, however, saying millions of ordinary citizens could be at risk of hunger and starvation.
South Korean President Roh Moo-hyun, whose country is technically still at war with the North, said strong pressure as well as dialogue might be needed to sway Pyongyang.
(Additional reporting by Lindsay Beck in Beijing, Jon Herskovitz in Seoul, Linda Sieg and Isabel Reynolds in Tokyo, and Canberra bureau)
