Rebel commanders scoffed when asked about reports of the delivery of 500 TOWs from Saudi Arabia, saying it was an insignificant number compared with what is available. Saudi Arabia in 2013 ordered more than 13,000 of them. Given that American weapons contracts require disclosure of the “end user,” insurgents said they were being delivered with Washington’s approval.

Equally graphic videos of new Russian firepower have been posted by pro-government fighters and journalists embedded with them.

Russian attack helicopters swoop low over fields, seemingly close enough to touch, then veer upward to unleash barrages of rockets, flares and heavy machine-gun fire. Explosions pepper distant villages, with smoke rising over clusters of houses as narrators declare progress against “terrorists.”

They appear to be using techniques honed in Afghanistan, where the occupying Soviet Army fought insurgents who were eventually supplied with antiaircraft missiles by the United States. Some of those insurgents later began Al Qaeda.

That specter hangs over American policy, and has kept Syrian insurgents from receiving what they most want: antiaircraft missiles to stop the government airstrikes that have been one of the war’s largest killers of civilians.

Now, they want them to use on Russian warplanes as well.

Mr. Saud, of Division 13, said he and other commanders renewed their requests for antiaircraft weapons 10 days ago to the liaison officers they work with in an operations center in Turkey.

“They told us they would deliver our requests to their countries,” he said. “We understand that it is not an easy decision to make when it comes to antiaircraft missiles or a no-fly zone, especially now that Syrian airspace is filled with jets from different countries.”