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Russia remains a key ally to Mr. Assad and one of the main weapon suppliers to Syrian military forces.

But Russian Foreign Minister Sergey Lavrov signaled Monday that Moscow has no plans to be drawn into a deeper military conflict over Syria’s civil war.

Reuters quoted Mr. Lavrov as warning Western powers against intervening in the war on grounds that doing so would violate international law.

“The use of force without the approval of the United Nations Security Council is a very grave violation of international law,” Mr. Lavrov said at a news conference. He said the use of chemical weapons in Syria was likely the work of rebels who wanted to derail plans by Washington and Moscow to hold peace talks on Syria’s future.

“If anybody thinks that bombing and destroying the Syrian military infrastructure, and leaving the battlefield for the opponents of the regime to win, would end everything — that is an illusion,” Mr. Lavrov said.