Europeans and the United States hope for a vote today for a UN resolution imposing sanctions on Iran's nuclear work.
But Russia's UN ambassador said further negotiations would prevent adoption for a day.
Vitaly Churkin said: "Maybe Saturday, yes, but clearly we will need tomorrow for further thinking and maybe further discussions of the draft resolution," adding that there were several unresolved issues in the UN Security Council draft.
Iran has vowed to continue its nuclear programme, which it says is for peaceful uses only, despite the resolution, even if it is approved by Russia.
Russia is building an $800 million light-water reactor for Tehran at Bushehr that is exempted in the resolution.
The draft demands Tehran end all uranium enrichment work, which can produce fuel for nuclear power plants as well as for bombs, and halt research and development that could lead to atomic weapons.
The sanctions include a ban on imports and exports of dangerous materials and technology relating to uranium enrichment, reprocessing and heavy-water reactors, as well as ballistic missile delivery systems.
The United States and European drafters of the resolution - Germany, France and Britain - held out hope a vote could still be called today after another round of discussions with Russia and China.