Fox News posted an exclusive report on Tuesday, based on information from three U.S. officials, that Russia is deploying the advanced SA-23 Gladiator anti-missile and anti-aircraft system to Syria.

It is difficult to imagine any target for the Gladiator other than cruise missiles and warplanes from the United States and its coalition partners. As one of the officials sarcastically asked Fox News, “Nusra doesn’t have an air force, do they?”

The Nusra Front is the (recently renamed) al-Qaeda franchise in Syria. The Islamic State does not possess an air force or cruise missiles, nor do any of the rebel groups seeking to overthrow the regime of Syrian dictator Bashar Assad.

As Fox News recalls, Russia sent S-400 air defense units to Syria after Turkey shot down a Russian plane along the Syrian border in November — a weapon dangerous enough to make American pilots wary of flying within its range.

The SA-23 would be a further Russian escalation, coming hard on the heels of the U.S. State Department announcing the end of bilateral communications with Moscow on Syria. Russian President Vladimir Putin retaliated by suspending a deal between the U.S. and Russia to dispose of weapons-grade plutonium.

According to Fox’s sources, the SA-23’s components arrived at Tartus, Russia’s Mediterranean naval base in Syria, over the weekend. They said the components are “still in their crates and not yet operational,” so their presence may be intended, for the moment, as a signal to Washington.

Military Times cites Russian Defense Ministry sources who say the components delivered to Tartus were more S-300 launchers, and they are intended solely to protect the strategically vital Mediterranean seaport.

The Russian Foreign Ministry announced on Tuesday that its embassy compound in Damascus was hit by shelling from rebel forces on Monday. The Russians blamed the attack on U.S. policy, saying it “provokes further bloodshed in Syria.” No casualties were reported from the incident.

Secretary of State John Kerry blasted the Russians on Tuesday at a European Union conference on Afghanistan.

“We acknowledge in sorrow – and, I have to tell you, outrage – that Russia has turned a blind eye to Assad’s deplorable use of chlorine gas and barrel bombs against his people. Together, the Syrian regime and Russia have rejected diplomacy and seem to have chosen instead to continue their pursuit of a military victory over the broken bodies, bombed-out hospitals and traumatized children of a long-suffering land,” Kerry said.