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While analysts said Mr. Ryabkov likely was referring to Israel in his remarks, rather than the U.S. or European nations, the posturing does not bode well for any efforts to draw Moscow into the Western diplomatic fold with regard to Syria.

“The Russian sale of the S-300 to Syria is a massive game changer,” said Anthony H. Cordesman, a senior national security Middle East scholar at the Center for Strategic International Studies think tank in Washington. “If it is more than a matter of words, and actual transfers take place, it virtually ensures that the U.S.-Russian talks will be meaningless.”

Such a development would also send “warning signals” to the West about Russia’s possible intent to make “similar arms transfer to Iran,” said Mr. Cordesman, who added that Moscow’s move may also “drag Israel into the Syrian fighting, and would sharply alter U.S. and allied ‘no fly’ capabilities if the Syrians can quickly absorb the system and make it effective.”

David Hartwell, Middle East analyst at IHS Jane’s, said that Russia’s use of the word “hotheads” was likely “a reference to Israel which has carried out two air raids in Syria in 2013 on what it alleged were Hezbollah-bound weapons convoys.”

The S-300 is an advanced, road-mobile, surface-to-air missile system that, like the U.S. Patriot, can shoot down other missiles as well as aircraft.

Russian authorities claimed the Syrian government’s contract for the system was signed several years ago — prior to the wave of Arab Spring protests against Mr. Assad that ultimately boiled into a full-blown civil war in 2011. Russia’s envoy to NATO, Aleksandr Grushko, said Moscow was acting “fully within the framework of international law” and “not doing anything that could change the situation in Syria.”

“The arms that we supply are defensive weapons,” Mr. Grushko said, according to the BBC.

Israeli Defense Minister Moshe Yaalon said Tuesday that Israel believes the Russian missiles have not yet been shipped, but that the Israeli military “will know what to do” if they are delivered.

Earlier this month, Israeli airstrikes hit suspected shipments of advanced Iranian missiles near the Syrian capital of Damascus that were purportedly intended for Hezbollah, the Lebanese militia that is fighting alongside Syrian regime forces.

Gen. Salim Idris, military chief of the Free Syrian Army, meanwhile, said Tuesday that rebel fighters under his command were at risk of losing control of a strategic town in western Syrian in the coming days unless he gets weapons quickly.

He said thousands of Hezbollah fighters are participating in an offensive against Qusair that began May 19, and that his fighters are outnumbered by more than 3-to-1.

“Time is a very important factor now in the battle in Qusair,” he said. “When they wait for a week [to send weapons], maybe Qusair will be under the control of Hezbollah. Then we don’t need their [the West’s] help, we don’t need their support.”

On Monday, Gen. Idris accompanied Mr. McCain into a rebel-held area in northern Syria for a meeting with about a dozen local commanders. In a comment on Twitter, the Arizona Republican on Tuesday praised the “brave fighters” battling Mr. Assad and renewed his call for the Obama administration to move aggressively to provide military aid to the opposition.

Shaun Waterman contributed to this article, which is based in part on wire service reports.