PARIS (Reuters) - The United States and 10 allies stepped up plans Thursday to intercept ships suspected of carrying weapons of mass destruction despite a warning from China that the move could be illegal.

Four nations, the United States, Australia, Japan and France, will send ships to the Western Pacific next week for an exercise simulating an interception, John Bolton, Washington's top arms control official, said after their talks in Paris.

Bolton rejected concerns that the initiative launched by President Bush in May risked breaking international law and said participating states had agreed a set of guidelines on how they would carry out interceptions of ships or aircraft.

"What we intend to do is consistent with national and international authorities," Bolton, undersecretary of state for arms control and international security, told reporters.

"Where we think we may have gaps in that authority, we are willing to consider seeking additional authorization," he said, adding one recourse could be to gain approval for action from the U.N. Security Council.

China, a neighbor of North Korea and which is being courted as a future member of the so-called Proliferation Security Initiative (PSI), earlier attacked the plan as legally dubious.

"The best way to prevent proliferation of weapons of mass destruction is through dialogue," Chinese Foreign Ministry spokesman Kong Quan told a news conference in Beijing.

"We understand the concerns of some countries about the proliferation of weapons of mass destruction...But many countries still question the efficiency and legitimacy of adopting this kind of measure."

The seven other nations in PSI are Britain, Germany, Italy, the Netherlands, Poland, Portugal and Spain.

A senior U.S. official in Washington said efforts to recruit new members of the initiative would begin "relatively soon." The next meeting of the group will be Oct. 9-10 in London.

Next week's "Pacific Protector" exercise is the first of 10 planned in coming months.

While the initiative is not specifically aimed at North Korea, few doubt its target is the reclusive communist state, which Washington and others accuse of making clandestine shipments of drugs, counterfeit cash and missiles.

Iran is another country of concern to initiative members.

Pyongyang said Monday it backed new dialogue with the United States, China, Russia, Japan and South Korea on defusing tensions but warned Washington it would step up its nuclear efforts if the U.S. did not drop its "hostile policy" toward it.

The "statement of interdiction principles" released after the Paris talks contains a commitment to take action "consistent with national legal authorities and relevant international law and frameworks, including the U.N. Security Council."

In a high-profile incident last December, Spain intercepted a North Korean ship in the Arabian Sea carrying Scud missiles for Yemen and handed over the vessel to the United States.

Washington allowed the ship to continue its voyage after concluding the missile shipment did not break any laws.

The senior U.S. official said the new initiative would not have applied to the December incident and that it was not "a 100 percent solution," in part because it would not cover interceptions of non-member ships on international waters.