U.S. Secretary of State John Kerry steps off his plane after arriving at Berlin Tegel Airport, ahead of a meeting with Germany’s Foreign Minister.

U.S. Secretary of State John Kerry said on Wednesday Russia’s military support for Syrian President Bashar al-Assad could raise the risk of confrontation with coalition forces fighting ISIS there.

In an interview with Italian newspaper La Stampa, Kerry said he had told Russian Foreign Minister Sergei Lavrov the United States was worried by Moscow’s military backing for Assad in Syria’s civil war, now in its fifth year.

“These actions could provoke a further escalation of the conflict and lead to the loss of more innocent lives, increasing the flow of refugees and risking a confrontation with the anti-ISIS coalition operating in Syria,” Kerry said.

He did not spell out what he meant by the coalition. But the United States and allies have been launching air strikes against positions held by the Islamist militant group in Syria, and in neighboring Iraq.

Israel has also raised concerns that Russian involvement on Assad’s side in the conflict in Syria could accidentally lead to confrontations between Russian and Israeli forces there.

Russia has built up naval infantry forces and heavy equipment including tanks and helicopters at Syria’s Latakia airbase, U.S. officials say, raising the possibility of air combat missions in Syrian airspace.

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