Secretary of State John Kerry says U.S. military experts believe the Russian planes sent to an air base in Syria are there for basic "force protection," intended to protect Russian soldiers and assets already on the ground.
But Kerry said Tuesday it is still not clear what Russia's long-term intentions are for boosting its military presence in Syria. He said if Russia is looking for a sphere of influence in the Middle East, it is going about it in a very odd way, by backing a Syrian government responsible for atrocities.
"Support for [Syrian President Bashar al-] Assad is support for a Shia minority that has dropped barrel bombs on its people, that has gassed his own people, war crimes, that has starved his people, tortured his people," said Kerry.
Russia has said it wants to defeat Islamic State, but has refused to join the U.S.-led coalition that shares the same goal.
Kerry said he hopes Russia and Iran - another major Syrian ally - are prepared to work for peace in Syria.
"But if they [the Russians] are there to shore up Assad and to simply stand there in a way that provides Assad with a continued sense that he doesn't have to negotiate, that I think it's a problem for Syria...and for everybody who wans to bring an end to this conflict which has gone on for too long now," he said.
The United Nations envoy on Syria launched another effort Tuesday to open peace talks on Syria. Staffan de Mistura announced the heads of four "thematic" working groups. The plan calls for Syrian parties to participate in discussions within the groups looking at political and legal issues, military matters and counterterrorism.
The 2012 Geneva Communique on Syria sets out a road map for a democratic transition and remains the basis for a peaceful settlement.
It includes an interim government ahead of a democratic election. But the U.S. has said Assad has no place in Syria's future.
Also Tuesday, the Pentagon confirmed that U.S. coalition airstrikes killed two top extremists.
David Drugeon, a French convert to Islam, was killed on July 5 in Syria. Drugeon was a bombmaker and terrorist trainer with the al-Qaida affiliated Khorasan Group.
Another U.S. coalition airstrtike near Tal Afar in northern Iraq killed Islamic State leader Abu Bakr al-Turkmani on September 10.