Iran's nuclear stance is leaving major powers with "little choice but to apply further pressure" on the Islamic republic, US Secretary of State Hillary Clinton said Thursday.

"Iran has provided a continuous stream of threats to intensify its violation of international nuclear norms," Clinton told reporters in London where she attended a day-long international conference on Afghanistan.

"Iran's approach leaves us with little choice than to work with our partners to apply greater pressure in the hopes that it will cause Iran to reconsider its rejection of diplomatic efforts," she said.

Clinton made her remarks a few hours after she swapped notes on Iran with four European counterparts in London -- Britain's David Miliband, France's Bernard Kouchner, Germany's Guido Westerwelle and Italy's Franco Frattini.

They discussed "where things stand and possible next steps both in New York (at the UN Security Council) and with respect to greater implementation of existing measures," a US official said.

They also heard a "short briefing" from Stuart Levey, the US Treasury's under-secretary for terrorism and financial intelligence, the official said on condition of anonymity.

Iran also came up when Clinton met one-on-one with Chinese Foreign Minister Yang Jiechi, officials said.

Britain, China, France, Germany, Russia and the United States have been negotiating with Iran over its nuclear programme amid concerns that the Islamic republic is secretly developing fissile material for nuclear weapons.

Under a UN plan, Iran -- which says it only wants nuclear know-how for civilian purposes -- would hand over most of its stocks of low-enriched uranium in return for French and Russian supplies of nuclear fuel enriched to the higher level required for a research reactor.

Thursday's meeting in London came a day after US President Barack Obama warned Iran of growing isolation if its pursues nuclear weapons.

It also followed talks in the British capital between Clinton and Russian Foreign Minister Sergei Lavrov, who said afterwards that Tehran cannot keep the world waiting forever in the standoff over its nuclear programme.