U.S. Secretary of State John Kerry says Russia's military support for Syrian President Bashar al-Assad "could provoke a further escalation of the conflict", as well claiming more lives and increasing the flow of refugees.
"If they are there to shore up Assad and to certainly provide Assad with the continued sense he doesn't have to negotiate, then I think it's a problem for Syria, and it's a problem for everybody who wants to bring an end to this conflict, which has gone on for too long," Kerry said.
Satellite images released this week showed a build up of Russian fighter jets, tanks and helicopters at Syria's Latakia airbase. Around 500 Russian troops, reportedly including dozens of troops who had been fighting in Ukraine are camped at the base. Thousands of reinforcements are reportedly on their way.
Meanwhile, the exodus of Syrian refugees into Europe has added urgency to finding a political settlement to the Syrian conflict.