Any move for international sanctions against Pyongyang, they said, would have to be done via the United Nations Security Council, but this had already been opposed by China and Russia and was expected to be rejected even by US allies Japan and South Korea.

North Korea meanwhile continues to say it will return to negotiations only if the conditions are right -- meaning the United States has to sweeten its offer on the table.

But Washington has ruled out giving any new incentives upfront to Pyongyang in return for disarmament, apart from endorsing a multilateral security guarantee and energy aid by the Stalinist state's neighbors.

US Secretary of State Condoleezza Rice, who held meetings last week with Chinese, South Korean and Japanese leaders, has warned that North Korea faced "problems" if it failed to return to the six-party talks and that Washington was looking at "other options."

This was seen by analysts as a strong hint that Washington might resort to sanctions against the hardline communist state, a move that was flatly rejected by veto-wielding powers Moscow and Beijing three years ago.

Japan is also unlikely to back any US move on sanctions until at least another round of talks is held to break the nuclear impasse, Hideshi Takesada, from Japan's National Institute of Defense Studies, told a forum in Washington.

"If we want to have decisive sanctions, China should join and also ROK (South Korea) should join.

"But China will say 'no' to discuss or argue the North Korean nuclear issue in the United Nations, China will say 'no' to participate in the proliferation security initiative and China will say 'no' to join in any economic sanctions with Japan and the United States," Takesada said.

He added that Rice could set the stage for the United States to seek UN support for its proliferation security initiative (PSI) against North Korea.

China is not part of the US-led PSI, aimed primarily at checking trafficking of weapons of mass destruction.

Host to the six-party talks, China is a key ally of and top aid provider to cash-strapped North Korea. But it has not been using full leverage against the reclusive state, accused of nuclear proliferation and other illicit activities.

"I'm not sure why China is not putting enough pressure on North Korea. I have my own suspicion but I think they have a responsibility in helping to resolve this problem," said Peter Brooks of the Heritage Foundation.

Three rounds of talks have been held among the two Koreas, Japan, the United States, Russia and China, but North Korea has refused to attend the fourth round scheduled last September and continues to shun the discussions.

"I don't think it is possible for South Korea and other countries to join US if it wants to impose sanctions on North Korea because they view that the US is not exhausting diplomatic means," said Park Kun Young of the Washington-based Brookings Institution.

He said Rice's warning of "other options" against Pyongyang "means bringing North Korea before the UN Security Council for possible sanctions."

Young said it was ridiculous for the United States to insist on a total ban on all nuclear activities in North Korea.

"The people of South Korea are wondering about what the United States means by (wanting a ban on) all nuclear programs. Does it include nuclear knowledge as well?."

Joel Wit, who served 15 years in the State Department in positions related to Northeast Asia, nuclear arms control, and weapons proliferation, said he was pessimistic of any breakthrough by the six-party process unless the United States revised its offer on the table.

He said "the fact is that North Korea is not under any pressure to reach agreement" on ending its nuclear weapons drive.

"Certainly not outside pressure and I would argue even internal pressure. They feel they could take their time," he added.

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