North Korea ignored a "strong warning" from China against staging its suspected nuclear weapon test, a UN diplomat said, as the Security Council planned emergency crisis talks for Tuesday.

The 15-nation council warned the North in a resolution passed last month that it would take "significant" action in the event of a new test or missile launch, and the body has called an emergency session for 9:00 am (1400 GMT).

The United States and South Korea, the council president for February, had both called for firm measures against North Korea if it stages a nuclear test.

China, the isolated North's closest ally, made special efforts to try to head off the bomb test, said the UN diplomat who has taken part in recent consultations.

"The Chinese gave the North Koreans a strong warning against carrying out a test as it became apparent that it was imminent," said the diplomat, speaking on condition of anonymity.

China has "special means of communications" with the entourage of North Korean leader Kim Jong-Un, the envoy added.

"What the North Koreans have done now is a big challenge to the Chinese.

There have been consultations in recent days, and in all likelihood China, Russia and the United States will quickly agree that tough action now has to be taken," the diplomat said.

The envoy added that sanctions may not be agreed upon on Tuesday but the "intention" would be made clear.

After North Korea's nuclear tests in 2006 and 2009, and North Korea's sinking of a South Korean warship in 2010, it took weeks for the Security Council to agree on statements and sanctions.

China has traditionally shielded its neighbor against international action, warning Western powers against any measures that could cause instability so close to its frontier.

The Security Council ordered tougher sanctions against North Korea last month for a rocket launch it staged on December 12, triggering a defiant pledge by Pyongyang to bolster its nuclear deterrent.

The Security Council added North Korea's state space agency, a bank, four trading companies and four individuals to its existing sanctions list.

China agreed to add to the resolution a threat of "significant action" if the North stages a nuclear test. (AFP)