World powers seemed divided over how to respond to the crisis over Iran's nuclear ambitions.
Britain, France and Germany, the so-called EU-3, have called for an emergency meeting on February 2-3 of the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).
The EU-3 call followed talks in London this week, which also included officials from China, Russia and the United States.
The IAEA could refer Iran to the UN Security Council, which has the powers to impose sanctions, but Russia and China are resisting European and US calls for such intervention.
"The Chinese and Russians are still formulating their own tactics of what to do at the board meeting," a British Foreign Office spokesman said.
Russia said sanctions were not the best way forward, while China called for a return to the negotiating table.
Russia and China, which have close trade and energy ties to Iran, have been reluctant in the past to pursue international action against Iran, but a senior British official said neither had objected to an IAEA session.
Russian Foreign Minister Sergei Lavrov said sanctions were "not the best and by no means the only way to resolve international problems."
He urged Iran to do more to cooperate with the IAEA: "Iran should do much more than it has done. Certain progress has been achieved in clarifying dark spots, but more is required of Iran."
Chinese foreign ministry spokesman Kong Quan said all sides at the talks in London had expressed concerns over Iran's actions.
"All the parties believed that Iran should return to the moratorium and to the diplomatic negotiation process," he said.
Brinkmanship
Iran, meanwhile, vowed reprisals against any crackdown, saying it would end UN inspections and fully resume nuclear fuel work if it is referred to the Security Council.
Ali Asgar Soltaniyeh, Iran’s ambassador to the Vienna-based IAEA, said Iran's decision to kick-start nuclear research was "irreversible".
The talks in London joining the five permanent Security Council members and Germany were called after Iran, defying the IAEA, broke the seals on a nuclear facility last week in order to resume research on uranium enrichment after a voluntary suspension.
Enriched uranium can be fuel for civilian nuclear reactors, but in a highly enriched form also the raw material for an atomic bomb.
An Iranian official said Tehran had written to Britain, France and Germany asking for fresh talks.
But British Prime Minister Tony Blair's spokesman said Tehran's offer was "vacuous."
He conceded there was a "great deal of diplomacy to pursue" in order to build an international consensus to bring Iran into line over a program which the US claims is hiding covert atomic weapons development.
Diplomats said if the case goes to the Security Council, a first step would be a statement urging Iran to honour existing IAEA demands that it cease nuclear fuel activities and resume talks on guaranteeing that its program is peaceful. Any sanctions would come later.
In Vienna, diplomats said that Iran still had a chance to back off and let diplomacy resume before the February IAEA meeting.
A diplomat close to the IAEA said that while the Iranians have taken seals off machines which manufacture nuclear reactor fuel, they have not started up the machines.
IAEA chief Mohamed ElBaradei on Sunday met with Iran's chief negotiator Ali Larijani, who lobbied for help in avoiding measures and offered the concession of allowing snap inspections, sources said.
The crisis over the nuclear fuel work in Iran, a major oil producer, has contributed to rising crude prices, which hit a three-month high on Tuesday.