Each of the permanent members of the UN Security Council has told Iran to drop plans for new nuclear activities or risk being hauled before the body for possible sanctions, the US said on Monday.
Although the US and European allies have been sending that message for weeks, China and Russia are now doing the same, State Department spokesman Sean McCormack said.
"We are working very closely with Russia, China and France and Britain on sending a clear message to the Iranians," McCormack said.
Those nations plus the US are the five permanent Security Council members. All are nuclear powers themselves and could individually veto any punishment the body might try to impose on Iran for pursuing what the US claims is a fraudulent and dangerous drive for nuclear technology.
The US is backing a stalled European effort to negotiate with Iran, and supports a separate offer from Russia to perform some of the most sensitive nuclear enrichment tasks on Iran's behalf.
Both initiatives would allow Iran to pursue legitimate civilian nuclear energy while reducing the risk that the same technology could be diverted to make weapons.
Iran says its nuclear ambitions are peaceful, while the US accuses Tehran of hiding a weapons program behind its drive for nuclear energy.
Iran has said it is ready to resume research on fuel production, which had been on hold so that a diplomatic solution to the crisis could be found.
There was no official word from Iran on Monday that it had resumed nuclear research, despite government pledges as recently as that morning that it would do so.
"Ultimately, given Iran's track record on seeking nuclear weapons under the cover of a civilian program, defying the international community, bobbing and weaving, obfuscating, that we're ultimately all going to end up in the Security Council on this issue," McCormack said.
There has been no single unified communication from Security Council members to Iran, such as a formal letter of warning, US officials said.
"I think that the Chinese are perfectly capable of delivering their own messages," McCormack said.
"What we have been doing, have done and will continue to do, is to continue to work with them, work with the Russians and others so that Iran receives a clear, consistent and unmistakable message from the rest of the world," he added.
French Foreign Minister Philippe Douste-Blazy called on Iran on Monday to immediately retract its decision to restart nuclear activities. He called Iran's intention to restart nuclear activities linked to uranium enrichment "reason for very serious concern."
"We call on Iran to go back on its decision without delay and without conditions," Douste-Blazy said.
Britain warned that the international community was ``running out of patience'' with Tehran, and Foreign Secretary Jack Straw said Iran had breached resolutions by the IAEA.
``There was no good reason why Iran should have taken this step if its intentions are truly peaceful and it wanted to resolve long standing international concerns,'' Straw said.