Russian President Vladimir Putin, one of five Security Council members with the power of veto, told US President George Bush that he had "deep doubts" about the justification for a strike, according to statement from the Kremlin press service.
Mr Bush had also telephoned French President Jacques Chirac and Chinese President Jiang Zemin, other leaders with reservations towards a strike.
The United States, Russia, France, China and the UK are the five permanent Security Council member countries. Major decisions by the Security Council, including those concerning Iraq, require the agreement of all of them.
British Prime Minister Tony Blair has emerged as Mr Bush's only unequivocal ally on the issue of Iraq, and says he is prepared for Britain to pay a "blood price" to secure its "special relationship" with the US.
On Thursday evening, US and UK planes took part in an attack on a major Iraqi defence facility in the west of the country.
Preparing the ground
The White House said the president had made the Friday phone calls because he wanted to "consult with friends and allies" on how to remove the threat posed by Saddam Hussein and his relentless acquisition of weapons of mass destruction".
Spokesman Ari Fleischer however declined to comment on remarks made by the Russian president during the telephone conversation.
Bush's diplomatic drive
Friday: Calls to leaders of Russia, China and France
Saturday: Talks with British PM
Monday: Meeting with Canadian PM
Thursday: Speech at the UN
Our Washington correspondent says Mr Bush would like to get his fellow presidents blessing for a new Security Council resolution which would present Saddam Hussein with an ultimatum - allow unfettered access to arms inspectors by a certain date or face serious consequences.
A spokeswoman for President Chirac said that during the conversation he had repeated his long-standing position on the issue: That any military action must be decided by the Security Council.
It is thought Russia is proving particularly hard to win over due in part to its financial links with Baghdad.
Russia wants to recover billions of dollars Iraq has owed since the Soviet era. Baghdad has also offered Russia a 10-year trade deal worth $40bn, which correspondents say is aimed at keeping it on side.
Tony Blair meanwhile is due to meet Mr Bush at the presidential retreat at Camp David this weekend. The US president is to outline his case to the UN Assembly General next week.
He faces stiff opposition from Arab nations, who on Thursday warned that military action to topple Saddam Hussein would "open the gates of hell" and that no Arab countries would join it.
'Invasion route'
Thursday's allied air strikes targeted the main air defence command centre for western Iraq, is part of the enforcement of the air-exclusion zones over the country, military sources said.
British defence sources say about 12 jets dropped bombs on western Iraq.
They say that fighter, tanker and radar support planes brought the total number of aircraft in the raid close to three figures.
However, the Pentagon later said some reports had exaggerated the size of the operation.
Brigadier-General John Rosa said: "It was bigger then the ones in the last two weeks, but we have done strikes of that size several times over the last 10 or 11 years."
He added: "We were fired upon when we were in that no-fly zone and we responded."
The command centre is potentially significant, analysts say, because the west is a possible invasion route and a centre for special forces operations.
Iraq earlier said warplanes attacked civilian targets in the Rutba district in Anbar province on Thursday, but reported no casualties.