The US Navy guided-missile destroyer USS Ross (DDG 71) fires a Tomahawk cruise missile in what the Defense Department said was a part of a strike against Syria on April 7. Reuters/obert S. Price/Courtesy U.S. Navy After having as many as 24 of its planes destroyed in a salvo of 59 cruise missiles from US Navy ships in the Mediterranean Sea on April 7, Syria has repositioned its jets to bases protected by Russian missile defenses, according to CNN.

"The Syrian air force is not in good shape," Defense Secretary Jim Mattis told reporters at the Pentagon, according to CNN. "It's been worn down by years of combat plus some ... significant maintenance problems."

Still, combined with the dozens of planes from his Russian backers, Syrian President Bashar Assad has an asymmetrical air advantage over his adversaries — rebel groups that have little more than a few anti-aircraft missile launchers.

The move to bases near Russian missile defenses provides Syria with a clear deterrent against further US strikes. Experts say Russia's S-300 and S-400 anti-air defenses can knock down Tomahawk cruise missiles, which were used in the April 7 strike.

Additionally, Russia has moved three warships to Syria's coast, further complicating the US's options should it launch another strike.

US officials have repeatedly stressed that they are "prepared to do more" against Assad's regime should more evidence of the use of chemical weapons in Syria appear, but the recent developments on the battlefield mean an engagement would be much more dangerous.

Russian S-400 Triumph medium- and long-range surface-to-air missile systems drive in a parade in Moscow in May. REUTERS/Sergei Karpukhin

Igor Sutyagin of the Royal United Services Institute an expert on Russian missile defense systems and strategic armaments, told Business Insider that the presence of Russian defenses didn't guarantee the safety of Syria's planes.

"One air defense battalion with an S-300 has 32 missiles," Sutyagin said. "They will fire these against 16 targets — maybe against cruise missiles they would fire a one-to-one ratio — but to prevent the target from evading, you always launch two ... but what if there are 50 targets?"

To further avoid detection, the US could use stealth aircraft like F-22s currently stationed in the theater.

A US Air Force F-22 Raptor flies over the Arabian Sea in support of Operation Inherent Resolve in January 2016. US Air Force photo by Staff Sgt. Corey Hook

Although the US could still carry out an attack against Syrian and Russian military targets, it would run a huge risk of killing Russian service members. The US warned Moscow ahead of the April 7 strike on Shayrat air base.

In this situation, where the target is Russian air defenses or planes on Russian bases, it's unclear if the Russians would back away from their hardware, and killing Russian service members would risk massive escalation.