Russia and France would process uranium for Iran under the deal
The leaders of the US and Russia have warned Iran that time is running out for talks over its nuclear programme.
President Barack Obama said Iran had failed "so far at least" to respond positively to a deal to send enriched uranium abroad for reprocessing.
Russia and France have offered to do this. Russia's Dmitry Medvedev said he was unhappy with the pace of talks, adding "other means" could be used.
The West suspects Iran wants to build weapons, but Tehran rejects this.
It says its programme is entirely peaceful.
Under the plan brokered by the UN nuclear watchdog, the IAEA, and agreed by Russia, the US and France, Iran would send about 1,200kg (2,600lb), or 70%, of its low-enriched uranium, to Russia by the year's end for processing.
Subsequently, France would convert the uranium into fuel rods for use in a reactor in Tehran that produces medical isotopes.
This is seen as a way for Iran to get the fuel it needs, while giving guarantees to the West that it will not be used for nuclear weapons.
Iran has raised "technical and economic considerations" with the International Atomic Energy Agency (IAEA) and has missed deadlines to respond.
"Unfortunately, so far at least, Iran appears to have been unable to say yes to what everyone acknowledges is a creative and constructive approach," Mr Obama said after meeting Mr Medvedev on the fringes of a summit of Asia-Pacific leaders in Singapore.
"We are running out time with respect to that approach," Mr Obama warned.
Mr Medvedev said: "Thanks to joint efforts the process has not stopped but we are not completely happy about its pace. If something does not work there are other means to move the process further."
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and the process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
He did not specify - but officials have previously mooted the idea of further sanctions.
"Our aim is clear - a transparent nuclear programme rather than a programme which causes others' concern," Mr Medvedev said.
Iran revealed the existence of a secret nuclear facility in September.
Iran says its nuclear programme is for purely peaceful purposes but the revelation of the existence of the new plant, near Qom, had increased fears in the West about Tehran's intentions.
Mr Obama's administration has set an end-of-year deadline for serious progress towards a comprehensive solution.
Correspondents say Russia and China are reluctant to agree to new Security Council sanctions, so a coalition of countries, including the EU, might take action themselves.
Iran is already subject to UN sanctions, including financial scrutiny and restrictions on arms imports, for keeping secret its uranium enrichment activities at its Natanz plant.