From Military.com:

In a warning to Russia, the Pentagon said Tuesday that the U.S. maintains the right to self-defense against advanced anti-aircraft systems sent to Syria by Moscow.

“This is something that we take very, very seriously — the safety of our aircrews,” Pentagon Press Secretary Peter Cook said in response to reportsthat Russia sent highly mobile S-300 anti-air missile systems (known to NATO as the SA-23) to its naval facility in the northwestern Syrian port of Tartus over the weekend.

“Those aircrews have the inherent right to self-defense,” Cook said at a Pentagon news conference.

Hillary Clinton via The Intercept:

“They’re getting more sophisticated thanks to Russian imports. To have a no-fly zone you have to take out all of the air defense, many of which are located in populated areas. So our missiles, even if they are standoff missiles so we’re not putting our pilots at risk—you’re going to kill a lot of Syrians,” she said. “So all of a sudden this intervention that people talk about so glibly becomes an American and NATO involvement where you take a lot of civilians.”

From the Guardian, “are US-led airstrikes on Syrians as bad as Russia’s?”:

It only attributes deaths with clear evidence so not all are accounted for, but its records hold the Syrian government and affiliated militia responsible for 92,000 civilian deaths, Russian forces for 3,412, Syrian opposition fighters, excluding Isis, for 2,470, and Isis for 3,078. It attributes 768 to the international coalition.