Iran needs to show "flexibility and sincerity" over its nuclear program, China's Foreign Ministry said Tuesday, as tensions rise ahead of the release of a new report on its atomic activities by a U.N. agency.

Spokesman Hong Lei said China is firmly opposed to any use of force against Iran to prevent it from acquiring an atomic weapon.

"China always holds that the Iranian nuclear issue should be properly solved through dialogue and cooperation," Hong told a daily news briefing.

"The Iranian side should also show flexibility and sincerity," he said.

U.S. officials say the government will use the International Atomic Energy Agency report as leverage in making its case to other countries that sanctions against Iran should be expanded and tightened and that the enforcement of current sanctions should be toughened.

China is Iran's biggest trading partner but has supported previous U.N. sanctions aimed at pressuring the Iranian government to suspend enrichment and start negotiations on its nuclear program _ which it has refused to do.

The report, which is expected to be issued Wednesday, will suggest that Iran made computer models of a nuclear warhead and include satellite imagery of what the IAEA believes is a large steel container used for nuclear arms-related high explosives tests, diplomats told the AP.

One senior U.S. official predicted it would stiffen the resolve of U.S. allies, particularly in Europe, to step up pressure on Iran to prove that its nuclear program is peaceful, as it claims.

Hong did not say whether China would support tougher sanctions on Iran or other new measures, but said China is firmly against the use of force in international affairs.

"At present, it is imperative to prevent new turbulence in the Middle East security situation," he said.