The UK, France and Germany plan to call for an emergency meeting of the UN's nuclear watchdog to discuss Iran, a possible first step towards sanctions.
Iran broke international seals at three nuclear sites
After talks in London on Monday, the European powers said they would ask the International Atomic Energy Agency (IAEA) to meet on 2-3 February.
Key UN members have agreed Iran must stop nuclear research, the UK says.
Western countries fear Iran aims to build nuclear weapons. Iran denies it, saying it wants civilian nuclear power.
It sparked a crisis last week by breaking international seals on three of its nuclear research facilities.
Russian President Vladimir Putin has hinted compromise is still possible.
He said Tehran had not rejected an offer for Iran's uranium enrichment to take place in Russia - which would make it harder for Iran to make nuclear weapons.
Iran's ambassador to Moscow, Gholamreza Ansari, praised the Russian offer as a constructive way out of the crisis, Russia's Channel One TV reported.
The US, UK, France and Germany have been trying to persuade Russia and China to support a hard line on Iran.
The six countries held a closed-door meeting in London on Monday.
After the meeting, the UK Foreign Office said the six had agreed Iran must stop nuclear research immediately.
Earlier, UK Foreign Secretary Jack Straw had said it was up to Iran to reassure the international community about its intentions.
"The onus is on Iran to act to give the international community confidence that its nuclear programme has exclusively peaceful purposes," he said.
Mr Straw said Western trust had been "sorely undermined by its history of concealment and deception".
After the talks, he said military action against Iran was not on the agenda, despite Washington's refusal to rule it out.
The BBC's world affairs correspondent, Nick Childs, says this is a high-stakes diplomatic stand-off and there will be a lot more talking, diplomatic visits and bargaining to be done in the run-up to the IAEA meeting.
The proposed meeting would be more than a month earlier than scheduled.
The head of the IAEA, Mohamed ElBaradei, has said he cannot confirm that Iran's programme has a peaceful objective.
IRAN'S NUCLEAR STANDOFF
Sept 2002: Work begins on Iran's first reactor at Bushehr
Dec 2002: Satellites reveal Arak and Natanz sites, triggering IAEA inspections
Nov 2003: Iran suspends uranium enrichment and allows tougher inspections
June 2004: IAEA rebukes Iran for not fully co-operating
Nov 2004: Iran suspends enrichment under deal with EU
Aug 2005: Iran rejects EU plan and re-opens Isfahan plant
Jan 2006: Iran re-opens Natanz facility
Western diplomats are thought to believe Russia can be persuaded to back a tough stance against Tehran, despite Russia's large investment in Iran's nuclear industry.
But China seems much more reluctant to threaten sanctions.
"All relevant sides should remain restrained and stick to resolving the Iranian nuclear issue through negotiations," a Chinese foreign ministry statement said on Monday.
There are other high-level discussions taking place in parallel with the talks in London.
EU foreign policy chief Javier Solana is meeting UN Secretary General Kofi Annan in New York, while Mr ElBaradei is hosting US Under-Secretary of State Robert Joseph in Vienna.
Iran says it will not be intimidated by international pressure and warns the threat of sanctions could result in higher oil prices.
Iran is the fourth largest exporter of crude oil. Prices rose in London to nearly $63 a barrel on Monday, with violence in oil-exporting Nigeria also seen as a factor.
Saudi Foreign Minister Prince Saud al-Faisal said in a BBC interview that the West was partly to blame for the crisis for allowing Israel to develop a nuclear arsenal.
He said nuclear weapons benefited no-one, and called for a nuclear-free zone in the Gulf.
The matter escalated last week when Iran broke the seals on three nuclear facilities, ending a two-year moratorium on atomic experimentation.
Mr ElBaradei has told Newsweek magazine that after three years of intensive work, he is still not able to conclude that Iran's nuclear programme is aimed purely at energy creation rather than the manufacture of nuclear weapons.
"If they have the nuclear material and they have a parallel weaponisation programme along the way, they are really not very far - a few months - from a weapon," he said.