She said Russian officials, in private talks, had recognised the need to act if diplomacy failed.
"We are in total agreement on all of that," Mrs Clinton told the BBC.
"And we are also in agreement that if our diplomatic engagement is not successful then we have to look at other measures to take, including sanctions to try to pressure the Iranians."
As a permanent UN Security Council member, Russia would need to back any fresh sanctions against Iran.
At the start of this month, Iran agreed at a meeting in Geneva to allow UN inspectors into a previously undisclosed nuclear site near its holy city of Qom, and to send low-enriched uranium abroad for enrichment to a higher level.
Mrs Clinton acknowledged to the BBC that Tehran had bought itself more time with this move.
But she said Iran had also made commitments which the Russians and the Chinese now expected them to fulfil.
However, wrapping up a visit to Beijing, Vladimir Putin said talk of sanctions was premature.
"I believe it's too early to speak of them," Russia's prime minister told media, reports AFP news agency.
"There is no need to scare the Iranians. There is a need to reach agreements."
After Moscow, Mrs Clinton headed for Kazan, capital of the religiously and ethnically diverse region of Tatarstan, east of Moscow.
The US has been trying to "reset" relations with Russia
Her five-day European trip included stops in Zurich, London and Belfast.
US President Barack Obama, who met Russian President Dmitry Medvedev in July, has pledged to reset relations with Moscow.
Mr Obama, for his part, has met a key Russian demand to scrap plans to deploy interceptor missiles in Poland and a radar station in the Czech Republic as part of a US missile defence system in Europe.
The US administration insisted it did not expect concessions in return.
But Washington has called on Moscow to support, or at least not oppose, the idea of tougher sanctions on Iran if it fails to live up to its international obligations.
The council wants Iran to end uranium enrichment and has approved three rounds of sanctions - including bans on Iran's arms exports and all trade in nuclear material.
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