Britain, Germany and France defied the US last month by offering Iran the prospect of sharing technology if it stops its disputed nuclear fuel enrichment programme and accepts tougher UN inspections.
Western diplomats said a joint letter by the big three European foreign ministers, the content of which has not previously been disclosed, was delivered to Tehran in early August despite intense lobbying by Washington.
It highlighted a gulf between the administration of US President George W. Bush and even its closest European ally, Britain, on whether to engage or isolate the Islamic republic.
The Europeans urged Iran to sign, implement and ratify the Additional Protocol to the nuclear Non-Proliferation Treaty (NPT) that provides for intrusive, short-notice inspections and to halt its uranium enrichment programme, which the West fears could be at the heart of a clandestine nuclear arms programme.
In return for compliance, the letter raised the prospect of cooperation on technology, without specifically pledging help with a civilian nuclear energy programme, the sources said.
"Washington did not consider it very helpful at all. They were worried it ran the risk of splitting Europe and America on this issue, and they talked to their friends and colleagues in Europe about that and attempted to dissuade them from sending the letter," a diplomat familiar with the exchanges said.
In Washington, State Department spokeswoman Brenda Greenberg said there was no immediate comment on the reported offer to Iran.
A French Foreign Ministry spokesman said he could confirm a letter had been sent to Iran, calling on it to sign the Additional Protocol. But he said it did not include any offer to cooperate on other issues.
European diplomats said they were disappointed there had not been a more specific reply from Tehran so far.
In Tehran, a leading cleric said on Friday Iran should consider quitting the NPT after the UN's nuclear watchdog -- the International Atomic Energy Agency (IAEA) -- set an Oct. 31 deadline for Iran to prove it is not seeking atomic weapons.
"What is wrong with considering this treaty on nuclear energy and pulling out of it? North Korea pulled out of it and many countries have never entered it," Ayatollah Ahmad Jannati told worshippers at Friday prayers at Tehran University.