US Secretary of State John Kerry declared Sunday that the hard work starts now on Iran, hours after world powers clinched a breakthrough interim deal on Tehran's controversial nuclear programme.

"Now the really hard part begins, and that is the effort to get the comprehensive agreement, which will require enormous steps in terms of verification, transparency, and accountability," he said at a joint press conference with British Foreign Secretary William Hague in London.

Iran agreed to curb its nuclear programme for the next six months in exchange for limited sanctions relief following marathon talks with the five permanent members of the UN Security Council plus Germany in Geneva that ended Sunday.

It is hoped a more permanent agreement will be reached later this year.

Both Kerry and Hague attended the Geneva talks before heading to London for their bilateral meeting.

Kerry vowed "to work together" with America's allies, saying "we'll start today, literally, to continue the efforts out of Geneva and to press forward."

He also said that US President Barack Obama "could not be more grateful" for the support of British Prime Minister David Cameron.

Cameron later said in a statement that the deal moved Iran "further away from getting a nuclear weapon" and called it "an important first step".

"We will continue to enforce sanctions robustly in order to secure a comprehensive and final settlement that fully addresses the real and substantive concerns of the international community," he said in a statement issued by his Downing Street office.

"Today's deal with Iran demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest," he added.

Kerry spoke ahead of separate talks on Libya and Syria with Hague.

The British minister praised the agreement as "a good deal for the Middle East and for the world".

"It's a very important opportunity for the future, and I think it vindicates the policy of pressure through sanctions and diplomacy through negotiations in which the United States and the United Kingdom have been strong partners for so long," he explained.

Hague earlier told the BBC that the deal did not refer to Tehran's "right to enrich" uranium, but added that a final agreement could allow "limited" enrichment for peaceful purposes.

"The phrase (right to enrich uranium) is not in the document," he said.

"What it says, is that as part of a comprehensive solution, if we reach that further stage of a comprehensive solution, Iran would be able to enjoy its basic rights to nuclear energy for peaceful purposes."