But Foreign Secretary Margaret Beckett was also downbeat about hopes for persuading the Islamic republic back to the negotiating table, which would avoid the growing threat of United Nations sanctions.

"I'm not anticipating any major decisions today," she said, as welcomed her counterparts from China, France, Germany, Russia and the United States to the talks in London.

Beckett would not be drawn on whether the six major powers would call for sanctions to be slapped on Tehran for refusing to suspend uranium enrichment activities, saying only: "That's something we shall have to discuss."

But she was downbeat about prospects of resuming negotiations with the Islamic republic, noting that efforts to persuade Tehran back to talks had failed since a UN resolution was passed at the end of July.

"Since then talks have continued to try and get negotiations going with Iran. So far they have not been successful and it looks rather as if they may not be successful," she said.

Washington has long led charges that Iran's nuclear power programme is a covert grab for atomic weapons, something that Tehran has hotly denied. It argues that the nuclear programme is purely for civilian energy purposes.

European Union foreign policy head Javier Solana has held a series of talks with Iranian negotiators in recent months, but pressure for an accord intensified after Iran failed to meet a UN deadline to halt uranium enrichment by August 31.

"What we want today is a full clear report from Javier ... so that we can all take stock of where we stand," said Beckett.

Solana acknowledged that diplomacy may have run its course, saying: "It is up to the six countries to decide whether the time has come to follow the second track -- referring the case to the (UN) Security Council," he said.

Russian Foreign Minister Sergei Lavrov, whose country has been cautious about US-led calls for sanctions on Tehran, reiterated Friday that the standoff still could be resolved through negotiations.

"All the measures that could be considered should be aimed exclusively at pushing Iran back to the negotiating table. That is the only way," Lavrov told journalists.

Washington has long spearheaded charges that Iran's nuclear programme is a covert grab for atomic weapons, something that Tehran has hotly denied. It argues that the nuclear programme is purely for civilian energy purposes.

The United States, and latterly Britain, are now leading moves to draft a resolution calling on the UN Security Council to impose sanctions which could go before the world body as early as next week.

A spokesman for British Prime Minister Tony Blair underlined the need for a united front over the crisis.

"What we know works is when we work together with our international partners," he said ahead of the start of talks, due around 5:00 pm (1600 GMT). A press conference was scheduled for around 6:45 pm.

But doubts emerged over whether decisions could be taken without Rice, who was delayed for two hours in Iraq.

"They are going to have as much of a meeting as they can," Rice's spokesman Sean McCormack said aboard the plane carrying her to London, via a stopover in Turkey.

He added that the ministers could talk again Monday or Tuesday by telephone before taking any decisions.

Britain's ambassador to the United Nations, Emyr Jones Parry, said Thursday that once the foreign ministers gave the green light, discussions would begin at the UN next week on a sanctions resolution against Iran.

Rice has said the United States wants a graduated series of sanctions, to be implemented through multiple UN resolutions that would ramp up pressure on Iran if it persists with its nuclear programme.

The first set of measures is expected to focus on preventing the supply of materiel and funding for Iran's nuclear or ballistic missile programmes.

Other steps could include asset freezes and travel bans on officials linked to possible Iranian weapons programmes.