WASHINGTON, Dec. 3 - In a new effort to pressure Iran to allow strict controls on its nuclear program, Britain and the United States are trying to persuade Russia and China to endorse their conclusion, derived from what officials call new evidence, that Tehran intends to build nuclear weapons, American and European diplomats said.

Until now, the effort to rein in Iran's nuclear program has occurred largely in the International Atomic Energy Agency, the United Nations nuclear watchdog, which has described a pattern of highly suspicious behavior by Iran. But the agency focuses on identifying possible diversions of nuclear material and has little weapons expertise.

The diplomats, who asked not to be identified to avoid any possible disruption of the delicate negotiations, say the new effort has been floated by Britain and endorsed by France and the United States, and seeks the declaration on Iran from the five major nuclear weapons powers that are the permanent members of the Security Council, which has the power to impose penalties.

The statement is the hoped-for result of arms specialists in China and Russia examining the evidence on Iran - including thousands of pages found on a laptop computer obtained by the United States last year - and concluding, as the United States, Britain and France have with varying degrees of certainty, that it points at least to an intent to build a weapon.

"If we could get China and Russia to agree that this bears all the hallmarks of a weapons program, it could have an enormous impact on Iran," said one senior European diplomat, because it might signal that if the issue reaches the Security Council, Iran could not count on Beijing or Moscow blocking action.

Russia and China have extensive energy and economic dealings with Iran and have argued that a confrontational approach will simply drive its government to walk away from international obligations on its nuclear program and oust the inspectors who are examining its facilities.

While Russia and China have declared that Iran should not be allowed to have nuclear weapons, they have resisted American efforts to get the nuclear agency to refer Iran to the Security Council for possible penalties. A resolution passed by the agency's member nations in September said the case should be referred, but specified no time frame, and none was established at a subsequent meeting last month.

Running parallel to the effort to declare that Iran is trying to "weaponize" its program, despite its denials, is a diplomatic effort to support a Russian proposal that Iran be allowed to enrich uranium for its nuclear reactors - but only on Russian soil. Russian officials said this would guarantee that the material could not be used secretly for nuclear weapons, because it would be enriched only to "reactor grade."

Many aspects of the Russian proposal remain unclear. An American official said it could involve Iranian investment in a company based in Russia, but with the actual enrichment done elsewhere in the country, with no Iranian participation.