LONDON — U.S. Secretary of State Hillary Rodham Clinton is using meetings in London this week to press ahead with imposing tough new international sanctions on Iran over its nuclear program.

A senior U.S. official said today that Clinton had raised the prospect of new U.N. Security Council sanctions against Iran if it continues to defy demands to prove its nuclear program is peaceful in talks with Russian Foreign Minister Sergey Lavrov.

They spoke openly about how to pressure Iran, including taking action within the U.N., the official said. The official, who spoke on condition of anonymity to describe a private meeting, said the discussion was constructive.

Russian news services reported after the talks that Lavrov showed lukewarm support for sanctions on Iran.

“Clearly, we can’t wait indefinitely, and our partners are talking about the need to discuss additional steps in the U.N.,” Lavrov was quoted as saying.

The official declined to offer further details but said Iran is an important element of Clinton’s trip and that she would be raising the same issue with the foreign ministers of China, Britain, Germany, France, Italy, Indonesia, Saudi Arabia, Turkey and the United Arab Emirates while they all are in London for a conference on Afghanistan.

Iran argues its nuclear program is aimed at creating a peaceful nuclear energy network to serve its growing population. The U.S. and other nations believe Iran’s nuclear program has the goal of creating atomic weapons.

The United States and its Western allies have been pushing for a fourth round of U.N. sanctions. But with Russia and especially China skeptical of any new U.N. penalties, they must tread carefully to maintain six-power unity on how to deal with the Islamic republic.