This story was updated at 1 p.m. Tuesday.
U.S. pilots flying over Syria and Turkey will be in range of advanced Russian-made surface-to-air defense systems, Pentagon officials said.
U.S. intelligence shows that Russia is following through on plans to send S-400 missile systems into its military base in Syria, the officials said.
With a reported range of up to 248 miles, those missiles could put at risk most U.S. combat aircraft flying over Syria.
Russian President Vladimir Putin ordered the S-400 missiles to Syria one day after a Turkish F-16 aircraft shot down a Russian Su-24 jet last Tuesday. Turkish officials said the Russian aircraft veered into Turkish airspace for less than 30 seconds and was fired upon only after repeated verbal warnings.
Russia's installation of the S-400s prompted a rare phone call on Monday between Marine Corps Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, and his Russian counterpart, the first time in several months the two top military officers have spoken directly.
The U.S. and Russia signed a narrow agreement Oct. 20 outlining safety protocols for sharing the air space over Syria and avoiding mishaps and misunderstandings.
Dunford wanted Russia to " reaffirm its commitment to the memorandum of understanding" for sharing the skies over Syria, Dunford told lawmakers Tuesday.
Defense officials also acknowledged that Russia has begun arming its combat aircraft over Syria with air-to-air missiles.
A Pentagon spokesman expressed concern about the surface-to-air missiles and other aerial targeting weaponry because it suggests the Russians are rejecting a call from American officials to focus on targeting the Islamic State group, also known as ISIL or Daesh.
“Certainly, we are aware of what they’re putting in there ... it’s a capability that we don’t see as being productive in the fight against ISIL,” Navy Capt. Jeff Davis, the Pentagon spokesman, told reporters Monday.
“Whether it’s putting air-to-air missiles on planes or putting in surface-to-air missiles, those are things that are clearly not about ISIL, as ISIL doesn’t have any aircraft,” Davis said. “So far it has not had any impact on our operations [over Syria], but it's certainly something we're aware of, particularly with the surface-to-air missiles."
The chief of U.S. Air Forces Central Command said last week that the deployment of S-400 missiles could have some impact on the mission. “It does complicate things a little bit, and we’ll put some thought to it, but we still have a job to do here, and we’re going to continue to do that job — to defeat Daesh,” Lt. Gen. Charles Brown Jr. told Air Force Times on Nov. 24.
The U.S. and its anti-ISIS coalition conducted three strikes on targets in Syria on Saturday and three more on Sunday, Defense Department data shows.
The U.S. recently sent air-to-air-capable F-15Cs to Turkey’s Incirlik Air Base near the Syrian border. Those aircraft are not slated for combat sorties over Syria but instead are assigned to a mission protecting Turkish airspace, officials say.
Those F-15Cs, deployed to Turkey in early November, so far have been limited to training missions and have not yet flown any combat air patrols, Davis said Monday.
Davis also said the U.S. supports Turkey’s account of the Nov. 24 incident with the Russian jet, noting that a review of U.S. intelligence indicates the jet did cross into Turkish airspace.