Moscow and Tehran have postponed the signing of a deal to supply fuel for Iran's new nuclear reactor in Bushehr.
Iran says its nuclear activity has solely peaceful motives
Both sides said an agreement would be made a day later than planned as negotiations in Tehran were continuing.
Under the deal, Iran has to return all its spent nuclear fuel to Russia in an effort to banish fears that Tehran will use it to make weapons.
Iran has insisted repeatedly that its nuclear programme has peaceful aims, and is solely to generate power.
Russian atomic energy chief Alexander Rumyantsev and his Iranian counterpart Gholam Reza Aghazadeh had been expected to sign the oft-delayed agreement on Saturday.
But officials said the two now hoped to sign the deal on Sunday, once differences over the timing of the first delivery of fuel were sorted out.
The US fears Iran could use spent fuel to make weapons, but Russia has rejected US pressure to cut nuclear co-operation with Iran, according to the BBC's Frances Harrison in Tehran.
At the very least, the US is concerned that the nuclear project could allow for the covert transfer of weapons technology to Iran, our correspondent reports.
At a meeting between Russian President Vladimir Putin and US President George W Bush in Slovakia on Thursday, both sides agreed Iran should not be allowed to develop nuclear weapons - and Mr Putin said he believed Iran had no intention of doing so.
The Russian ambassador in Tehran, Alexander Maryasov, said the signing of the agreement would leave "no grounds to state that Iran may use nuclear fuel for other than peaceful purposes", the Itar-Tass news agency reports.
Mr Rumyantsev is also expected to visit Bushehr to inspect the $800m reactor, built with Russian help.
There is already talk of Russia being awarded a contract to build another reactor at the site, our correspondent says.
The signing of the nuclear-fuel deal has been delayed several times in the past over apparent technical and financial issues.
According to the AFP news agency, Iran was initially reluctant to agree to Russian demands for all spent fuel to be returned, citing the risks involved in transporting it.
But Russia insisted on the guarantee to ensure no spent fuel was diverted for the manufacture of weapons.