Clinton will also will seek to enlist more countries such as Indonesia to bring pressure on the Iranians, a senior U.S. official said.
"It is fair to say that Iran will be a fairly important element of her trip here," said the official, speaking on condition of anonymity.
Clinton's London trip includes a Wednesday meeting on assisting Yemen root out al Qaeda militancy and a Thursday conference on Afghanistan, where the United States is leading a major military build-up in the fight against Taliban insurgents.
But Iran, a regional power locked in a deepening stand-off with the rest of the world over its nuclear programme, looms in the background as a potential flashpoint.
Tehran has ignored U.S. President Barack Obama's Dec. 31, 2009 deadline to respond to an offer of economic and political incentives in return for halting its nuclear programme, which Western nations fear is geared to producing nuclear weapons.
The countries negotiating with Iran include the five permanent members of the U.N. Security Council, the United States, Britain, France, Russia and China, plus Germany.
Clinton was expected to meet in London with the Europeans to discuss Iran, as well as to talk with the foreign ministers of Russia and China, which appear less eager to consider a quick push toward further sanctions.
The official said there was no formal move among the foreign ministers to discuss the next steps on Iran after a meeting in New York earlier this month at which senior diplomats took no decision on further sanctions.
But he said the meetings would give Clinton the chance to continue the broader discussion of what might be included in any future U.N. sanctions as well as how existing sanctions on Iran are being implemented.
"Those conversations are ongoing and they'll continue for some time," he said.
The official said the United States still had no word on whether Iranian Foreign Minister Manouchehr Mottaki would attend Thursday's Afghanistan conference, but that there were no plans for the U.S. delegation to meet him even if he does show up.
Clinton did use the London visit to try to broaden international pressure on Tehran, talking to Indonesia's foreign minister in hopes of adding the world's most populous Muslim country to those urging Iran to come clean.
"Indonesia is a country that she believes is growing in stature in the global community, and adding its voice to the view that Iran is in breach of its international obligations and should come into compliance with them is important," the official said.
Negotiating a new U.N. sanctions resolution could likely take months, Western diplomats predict. But several Western diplomats said they hoped the process would be over by May, when signatories of the 1970 nuclear Non-Proliferation Treaty gather to discuss ways of upgrading the treaty to deal with threats like Iran and North Korea. (Reporting by Andrew Quinn; Editing by Jon Hemming)
