SEOUL, Oct 11: A defiant North Korea warned on Wednesday that it would regard harsh sanctions over its nuclear test as a declaration of war and threatened further trials if the United States kept up its pressure.

An apparent offshore earthquake early in the morning sparked Japanese media reports about a second nuclear test, which were later dismissed.

As the UN Security Council weighed what action to take, Pyongyang's number two and its foreign ministry warned of ‘physical’ measures if it was hit with the kind of sanctions proposed by Washington and Japan.

The rhetoric underscored the standoff between the international community and the secretive nation. The chance of sanctions grew after even the North's main ally China said it would support punitive action.

"If the US continues to harass and put pressure on us, we will regard this as a declaration of war and will take a series of physical countermeasures," said a foreign ministry statement carried by the North's official Korean Central News Agency.

It did not elaborate on the measures, but insisted it was still ready for talks to improve security and stability on the Korean peninsula. "We are ready for both dialogue and confrontation."

Statements from the foreign ministry are seen as carrying more weight than comments on state media. It was the ministry which announced on October 3 that Pyongyang planned a nuclear test.

The message was reinforced by Kim Yong-Nam, who as head of the North Korean Supreme People's Assembly is effectively the regime's number two.

"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 said in an interview with Japan's Kyodo News.

He added: "The issue of future nuclear tests is linked to US policy toward our country."

HRW WARNING: Human Rights Watch warned against any action that would hit emergency aid supplies in North Korea, with a third of the nation relying on foreign aid for food.