WASHINGTON — The United States, which is trying to bring Syrian rebels and the Syrian government to the negotiating table, is now increasingly worried that Russia plans to sell a sophisticated air defense system to Syria, American officials said Wednesday.

Russia has a long history of selling arms to the Syrians and has a naval base in the country. But the delivery of the Russian S-300 missile batteries would represent a major qualitative advancement in Syria’s air defenses. The system is regarded as highly effective and would limit the ability of the United States and other nations to operate over Syrian airspace or impose a no-fly zone.

It is also able to track and fire missiles at multiple targets, including aircraft and some missiles.

“There are concerns that this might happen,” said a senior United States official who spoke on the condition of anonymity, referring to the possible delivery of the S-300. A Western intelligence service has also warned that the Russians may soon send S-300 air defense batteries to Syria, said another American official who asked not to be identified because he was discussing intelligence reports.