The administration of US President George W. Bush said a proposal by Iran for nuclear negotiations fell short of UN demands that it cease uranium enrichment, and the US had begun plotting unspecified "next moves" with other governments.
At the same time, Iran contended it had offered "positive and clear signals" to resolve the dispute over its nuclear program.
Efforts by the US and other nations could lead to UN sanctions against Iran unless it reverses course and agrees to a verifiable halt to enrichment activities that can be central to making nuclear weapons.
France took a firm and quick stand on Iran's proposal. Foreign Minister Philippe Douste-Blazy said the Iranians must suspend uranium enrichment if they want to return to negotiations.
However, Russia's foreign ministry, evidently ambivalent, said it would continue to seek a negotiated solution. And China appealed for dialogue, urging "constructive measures" by Iran and patience from the US and its allies.
The US State Department, in a terse statement on Wednesday, acknowledged that Iran considered its proposal to be a serious one.
"We will review it," the statement said in what appeared to be a conciliatory gesture to a government it regularly denounces as a sponsor of terror.
But the statement went on to say that Iran's response to a joint offer of US and European trade and other benefits if the enrichment program was halted "falls short of the conditions set by the [UN] Security Council" -- full and verifiable suspension of all uranium-enrichment activity.
"We are consulting closely, including with other members of the Security Council, on next steps," it said. The UN has set a deadline of next Thursday for a formal reply by Tehran.
Bush met with US Secretary of State Condoleezza Rice at the White House and then discussed Iran's proposal in a telephone call with UN Secretary-General Kofi Annan.
The call was initiated by Annan, White House spokeswoman Dana Perino said.
The administration has cautioned Iran that it will seek sanctions in the Security Council if Tehran does not step enriching uranium.
Administration officials have refrained from outlining what punishment they might have in mind. It could include economic or political penalties, perhaps international curbs on trade.
Rice, meanwhile, telephoned Javier Solana, the senior EU diplomat who oversees exchanges with Iran. No account of their conversation, nor of her meeting with the president, was provided.
By not rejecting Iran's proposal outright, the US indicated there may be a basis for dealing with long-held concerns that Tehran is developing nuclear weapons, an allegation the Iranians deny.
"The diplomats are continuing to look at it," Perino said. "We're working with our allies."
Iran's offer, which it portrayed as a major advance, appeared to be aimed at least partly at dividing the Security Council members with vetoes -- the US, Britain and France on one side and Russia and China on the other.
Analyst Ilan Berman, vice president for policy at the American Foreign Policy Council, said sanctions can work because Iran's economy is vulnerable on several levels.