Russia has said military intervention in Syria would have "catastrophic consequences" for the region.
The warning from Moscow came as the US and its allies considered launching strikes on Syria in response to deadly attacks last week.
US Secretary of State John Kerry said on Monday there was "undeniable" evidence of a chemical attack.
UN chemical weapons inspectors are due to start a second day of investigations in the suburbs of Damascus.
The UN team came under sniper fire as they tried to visit an area west of the city on Monday.