The United States and China have made a deal under which the UN Security Council will expand existing sanctions against North Korea for staging a ballistic missile test, envoys said Friday.

The deal was struck after weeks of intense negotiations following the December 12 launch. The talks have involved US Secretary of State Hillary Clinton and her Chinese counterpart Yang Jiechi, according to envoys.

China is studying a proposed Security Council resolution that is expected to be quickly sent to all 15 members and could be passed next week, diplomats said.

The United States has sought a Security Council resolution with tough new sanctions against the nuclear-armed North for the rocket launch.

But China wants to shield its ally against new action on top of sanctions ordered after its nuclear tests in 2006 and 2009. It wanted only a lower level council statement.

"This is a compromise," said one diplomat with knowledge of the negotiations. "The United States will get a formal resolution and widening use of the existing measures. China can say that it has avoided new sanctions."

Another envoy said: "It is just awaiting China's final approval."

Both countries want any resolution passed before South Korea takes over the presidency of the Security Council in February, envoys said.

US and Chinese diplomats made no immediate comment on the negotiations, which were led in New York by US Ambassador Susan Rice and her Chinese counterpart Li Baodong.

But the US administration has come under strong pressure from South Korea and Japan to stand firm in insisting on significant action against the North, said one UN diplomat.

"Washington wants a strong message to be sent to Pyongyang so this has involved Secretary Clinton and the Chinese minister," added the diplomat. "It has been conducted at the highest level."

The Security Council agreed on a presidential statement, with lower standing than a binding resolution, after North Korea staged a failed rocket launch in April.

The statement called for a tightening of the existing sanctions and warned of new measures if North Korea staged a new rocket launch. "That warning and fears that North Korea could stage a nuclear test may have spurred China to take this action," said the UN diplomat.

North Korea said the December launch was aimed at placing an earth observation satellite in space. The launch was considered a major boost to the isolated state's young leader Kim Jong-Un.

North Korea has meanwhile sent a letter to the United Nations warning that the new US strategy of increasing its military and diplomatic focus on the Asia-Pacific region increased the risk of a nuclear war.

The North Korean foreign ministry statement sent to the UN said the United States wants to set up a NATO-style military bloc in Asia, which "would inevitably trigger off a countervailing force from other countries which are placed under the target of this bloc."

This would lead to "a revival of the Cold War and increased danger of a nuclear war beyond any measure," said the statement.