David Cameron has urged Russian president Vladimir Putin to maintain a "joint approach" over Iran amid concerns a breakthrough in nuclear negotiations could be undermined by a Moscow trade deal.
Under an historic agreement struck in November, Tehran agreed to halt some uranium enrichment and allow regular inspections in return for an easing off of some sanctions such as in petrochemicals and gold.
Last week it was reported that Russia had opened negotiations with Tehran on an lucrative "oil for goods" deal that it is feared could remove the pressure of remaining sanctions.
Washington is reported to be exerting significant pressure on the Kremlin not to proceed.
The Prime Minister telephoned Mr Putin to discuss the "steps needed to secure a long term comprehensive solution that would ensure Iran's nuclear programme will be exclusively peaceful", Downing Street said.
Russia is part of the group of E3+3 nations - which also includes the US, China, Britain, France and Germany - which has been leading negotiations over Iran's nuclear ambitions.
"The Prime Minister emphasised the importance of maintaining a joint approach on this issue and ensuring that the bulk of sanctions, including core measures on oil, remain in place," a spokeswoman said.
Mr Cameron - who is not attending the Winter Olympics which get under way in the Russia city of Sochi this week - also wished Mr Putin "a safe and successful" Games.
They spoke about assistance being given by the UK to Russian efforts to counter any terror threats to the Games.
Gay rights campaigners have urged the PM to boycott the Sochi games in protest at Mr Putin's introduction of laws banning homosexual "propaganda".
But Downing Street insists he is not travelling because of diary pressures and points out that no serving British prime minister has ever attended the Winter Olympics.
On the search for an end to the Syrian civil war, Mr Cameron pointed to the " need for progress towards a transitional authority and to continue promptly with the process of destroying Syria's chemical weapons".
Moscow is a key ally of the Damascus regime of Bashar Assad.
They also agreed to "work together to encourage the Ukrainian authorities to pursue a path that avoids further violence", Number 10 said.