U.S. Secretary of State John Kerry has said Syrian President Bashar al-Assad has to go but the timing of his departure has to be decided through negotiations.
Speaking after talks with British Foreign Secretary Philip Hammond in London on September 19, Kerry called on Russia and Iran to use their influence to convince Assad to negotiate.
He said there was an urgency to renewing efforts to find a political settlement to end the four-year war and worsening refugee crisis in Syria.
Elsewhere, a powerful Syrian rebel group has claimed that it fired several rockets at an airport on the coast said to be used by Russian troops.
In a video posted on September 18, members of the Jaish al-Islam (Army of Islam) warn the Russians that they will not enjoy peace in Syria.
Earlier this week, new satellite imagery showed the recent arrival of Russian tanks, armored personnel carriers, and other military equipment at an air base in Syria's coastal Latakia Province, confirming reports by U.S., Israeli, and other officials of a Russian military buildup.