The British government has expressed concern and "huge regret" that Iran has reopened its nuclear research plants.
Iran's nuclear ambitions have been causing concern for several years
Foreign Secretary Jack Straw said it was a "serious situation" and vowed it would be "top of the agenda" at an EU foreign ministers' meeting this week.
Mr Straw said he was "confident" the crisis would be resolved by peaceful means rather than military action.
Western countries fear Iran's nuclear programme could be used to make atomic bombs, but Tehran denies such a goal.
"This is a matter which has to be resolved by peaceful means, but it will involve a good deal of diplomatic and other pressure on Iran," Mr Straw told a press conference.
He confirmed he would meet French Foreign Minister Philippe Douste-Blazy, Frank-Walter Steinmeier of Germany and Javier Solana, the EU foreign policy chief, on Thursday.
"We have shown our good faith in Europe over the last two and a half years. I do not believe we could have done more to reach out to the Iranians," he said.
Mr Straw added it was a matter of "huge regret" that Iran had renewed its nuclear programme, but said he hoped they would reconsider.
The International Atomic Energy Agency (IAEA) said Iran had removed international seals on one of its nuclear plants on Tuesday.
The Middle Eastern country confirmed it intended to restart its research immediately.
Mr Straw said earlier Iran's decision breached IAEA rules as well as a 2004 agreement Iran had signed with the UK, France and Germany.
Downing Street reflected Mr Straw's concerns, a spokesman saying the international community was "running out of patience" with Iran.
"There's a real impatience at what is seen as Iran's defiance of the international community," said the spokesman.