Mr. Kerry’s remarks, his sharpest criticism of Russia since taking office in February, signaled a growing frustration with its continuing support for Mr. Assad’s government, including the delivery of various types of weaponry as part of what the Russians have said are longstanding arms contracts. He said the missiles in particular threatened regional stability because of the threat they would pose to Israel, which has all but said it would respond militarily to stop the shipments.

“Whether it’s an old contract or not, it has a profoundly negative impact on the balance of interests and the stability of the region, and it does put Israel at risk,” Mr. Kerry said at the State Department. “It is not, in our judgment, responsible because of the size of the weapon, the nature of the weapon and what it does to the region in terms of Israel’s security.”…

“We will learn very quickly whether or not they and others are acting in good faith” to negotiate a settlement, Mr. Kerry said in remarks that also criticized Hezbollah and Iran for intervening directly in the fighting on Mr. Assad’s behalf. “If they’re not, the world will know it.”