U.S. Secretary of State Hillary Clinton said on Tuesday Washington was worried Russia may be sending attack helicopters to Syria and described as "patently untrue" Moscow's argument that its arms transfers to Syria are unrelated to the conflict there.
"They are providing arms and weapons to the Syrian opposition that can be used in fighting against the Damascus government," he said on Iranian state television, speaking through an interpreter.
Russia is one of Syria's principal defenders on the diplomatic front and, as a permanent member of the U.N. Security Council with the power to veto resolutions, has stymied efforts by Western powers to pressure President Bashar al-Assad into stepping down.
Lavrov said Russia's position was based on concern for the Syrian people and the country's integrity, rather than personal preference for Assad.
"I have announced time and again that our stance is not based on support for Bashar al-Assad or anyone else ... We don't want to see Syria disintegrate."
Russia is resisting Western and Gulf Arab pressure to take a harder line against Assad, rejecting calls for sanctions and proposing a conference bringing together global and regional powers including Iran.
"The Islamic Republic of Iran has announced many times: the issue of Syria needs to be dealt with in Syria by Syrians, not through the interference of others"
The United States says it does not believe Iran, Assad's closest regional ally, is ready to play a constructive role in Syria, where the United Nations says government forces have killed more than 10,000 people since March 2011.