Russia is reportedly increasing the number of S-400 surface-to-air missile systems in western Syria, where it operates maritime and air forces.
Two mobile missile batteries have been delivered to Khmeimim air base in Latakia province, while another two went to Tartus naval port on the Mediterranean Sea, according to RT, the Russian government-funded news outlet.
Whether this poses significant or increased threat to U.S. warplanes operating in the country for the fight against the Islamic State is unknown. The missile batteries' final locations have not been disclosed.
U.S. Air Forces Central Command would not comment on the reported SAMs deployment or on any extra precautions it might take for fighter and drone aircraft operating in the country.
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"AfCent always remains vigilant and committed to the defense and protection of U.S. personnel, our coalition partners and assets. We posture our forces as necessary to counter potential threats," Capt. AnnMarie Annicelli told Military.com in a statement Thursday.
The U.S. generally operates east and north of Euphrates River, while Russian and Syrian forces operate in the west and south.
Russia and the U.S. have an established "deconfliction zone" -- an area in which they have agreed not to operate. The zone previously applied to airspace but now includes ground territory, a defense official told Military.com last year.
Russia first deployed the S-400, known as the "Triumf," to Syria in 2015, the same year it entered the battlespace. The advanced system can carry multiple, short- to very long-range missiles with a variety of sensor systems. Exactly how many Russian-made SAMs are in Syria is unknown.
In June, Russia condemned the U.S. shootdown of a Syrian Su-22 fighter bomber over Syria, saying it would track aircraft of any kind near its airspace in Syria with surface-to-air missiles and claiming the U.S. failed to use the established deconfliction line between the militaries before the incident.
Months earlier, a top U.S. Air Force general stressed more discussions with Russia were needed as the battlespace in Syria began to shrink and become more complex.
"It's a dense surface-to-air threat in portions of Syria," Gen. Herbert "Hawk" Carlisle, then-commander of Air Combat Command, said last February.
Aircraft must be ready at all times to act, he said, because "the [surface-to-air missile] systems the Russians and Syrians have over there are active systems."
He clarified that while the SAM systems are operative, they're not necessarily painting U.S. aircraft with target tracking radars "to any great extent that I know of."
The latest S-400 deployment comes as Russia's Ministry of Defence has vowed to sell the weapons system to other countries. In addition to Turkey, nations expressing interest include Qatar, Saudi Arabia and Syria, Newsweek reports. India is also in negotiations to purchase five Triumf systems for roughly $5 billion, Defense News reports.