The British Foreign Office was refusing to confirm the talks, but sources said plans were going ahead for Britain, China, France, Russia, the United States and Germany to try to thrash out a joint stance.

The six powers agreed to set a new deadline this week for Iran to comply with a UN resolution demanding it freeze its uranium enrichment programme, which Washington and others believe is a cover for developing nuclear weapons.

Otherwise, there is growing talk of imposing sanctions on the Islamic republic, which has repeatedly refused to commit itself to suspending enrichment despite the mounting diplomatic pressure.

"We have been following one track, that of negotiations, but this has come to nothing. So we now have to turn back to the other track," said a diplomatic source in Brussels.

But the source added: "At some point we hope to return to the negotiating track. We have to leave that path open because in the long term it is the only way of resolving this problem."

According to a European diplomat in London, the talks including US Secretary of State Condoleezza Rice -- on her way back from a Mideast trip -- will begin late afternoon, followed by an evening session over dinner.

After the meeting, British Foreign Secretary Margaret Beckett is expected to sum up the conclusions on behalf of the so-called P5-plus-1 group -- the five permanent members of the UN Security Council plus Germany, the source said.

The meeting was expected to agree that Iran "does not want to negotiate and draw conclusions from that," the source told AFP.

Rice, who is pressing for sanctions on Iran, warned this week that the credibility of the international community is on the line and time is "running out."

Iran insists its nuclear programme is solely for peaceful energy needs and has vowed it will not suspend uranium enrichment, defying warnings of sanctions from world powers.

Iran's chief nuclear negotiator Ali Larijani and EU foreign policy chief Javier Solana have staged four rounds of talks on Iran's nuclear programme but have failed to make a breakthrough.

According to the diplomatic source in Brussels, the Europeans are willing to talk about sanctions, but insist they must be "proportionate, progressive and reversible."

"If we decide to start to introduce sanctions, they must be very specific sanctions, very concrete, on technologies which can be used in the nuclear sphere," he added.

Solana's spokeswoman Cristina Gallach underlined that the current diplomatic efforts were different from a European initiative in 2004-2005, which ultimately failed.

"The offer of November 2004 was only one and a half pages long and only had the Europeans' signature, while the June 2006 offer has the signature of the whole Security Council and Germany.

"The Americans are very much on board, it is a truly robust international offer."

The decision to hold the London talks, which have been rumoured for some days, was confirmed by a Russian foreign ministry spokesman in Moscow, quoted by the Interfax news agency.

But the British Foreign Office has so far refused to confirm them. A spokesman said Thursday morning that nothing had yet been decided.