Patrick J. McDonnell and Rima Marrouch / Los Angeles Times

BEIRUT — Thwarted by aerial attacks, Syrian rebels appear to have begun systemically targeting government air bases and aircraft, while trying to capture antiaircraft missiles that can shoot down helicopters and fighter jets.

An opposition group reported Saturday that insurgents had captured an air defense base in eastern Syria, close to the Iraqi border, and that battles were raging near a military airport in the area. News reports cited opposition sources as saying that some antiaircraft missiles were seized.

It was at least the third time in the last week that rebels reportedly targeted government air installations. The others were near the northern cities of Aleppo and Idlib, close to the Turkish border. If true, the operations would appear to demonstrate an elevated degree of coordination among various rebel brigades, which often operate autonomously.

The attacks seem to be directed at bases from which aircraft take off to strike opposition-held territory.

“When you can’t hit the airplanes when they are flying, you will make sure they can’t take off,” said a rebel representative in Idlib province who goes by the nickname Abu Bashir. “There are not enough antiaircraft weapons and missiles, so targeting air bases ... is a natural choice.”

The government of President Bashar Assad rarely deployed fighter jets and helicopter gunships in the early days of the conflict, now in its 18th month. The military initially relied heavily on tanks, artillery and ground troops. But as insurgent forces grew in strength and attacks became more audacious, the government has unleashed its considerable air assets.

Rebels complain bitterly about a lack of surface-to-air missiles that could help neutralize military aircraft. There have been repeated reports that the opposition has received some supplies of such weapons, either provided by overseas suppliers, captured from government stocks or purchased on the black market. But there has been no indication of widespread deployment of shoulder-fired surface-to-air missiles in Syria.

Still, in the last week, the opposition has reported downing several aircraft, including a helicopter that was seen in a video Monday falling to the ground in flames in Damascus. On Thursday, another opposition video posted on the Web appeared to show a downed fighter jet in Idlib province, complete with a pilot apparently parachuting from the aircraft.

The authenticity of the videos could not be corroborated.

On Saturday, the new U.N. envoy to Syria issued a warning to the government. “I call on parties inside Syria to halt the fighting. Undoubtedly, this call is primarily directed to the government,” the envoy, Lakhdar Brahimi of Algeria, said in New York. “The need for change is urgent and necessary.”

Also Saturday, the government and rebels accused each other of a new round of extrajudicial executions. Such charges have become an almost daily feature of the conflict.