During her visit to Moscow, Clinton plans to press for a strong commitment from Russia for tough new sanctions against Iran.
US officials said Iran will be at or near the top of Clinton's agenda when she meets Russian President Dmitry Medvedev and Foreign Minister Sergey Lavrov on Tuesday.
Russia and China have long balked at imposing new sanctions on Iran if it fails to come clean about its suspect nuclear program, but Medvedev hinted the Russian position might be shifting after Teheran disclosed a previously secret uranium enrichment site near the holy city of Qom.
But US officials believe it will be a hard sell to convince the Russians on fresh penalties since Iran agreed to allow UN inspectors to visit the Qom site and has agreed, in principle, to send most of its low-enriched uranium to Russia for reprocessing.
Iran agreed to allow inspections of the Qom site following talks last week between Iran's chief nuclear negotiator and diplomats from the US, Russia, China, Britain, France and Germany. The Iranians were given time to decide whether to accept a package of incentives in exchange for Iran's compliance with international demands to suspend its uranium enrichment or face new sanctions.
The Obama administration is anxious not to let up on the pressure and Clinton will be looking for Russian expressions of support for sanctions and other penalties should Iran continue to refuse by the end of the year, the officials said.
"Iran has not bought an indefinite delay and we want them to know that," said one official, who like the others spoke on condition of anonymity to preview Clinton's talks.