BEIRUT, Lebanon — Syrian insurgents seized control of a northern military airfield on Tuesday and captured usable warplanes for the first time in the nearly two-year-old conflict, according to rebels and activist groups. The development, if confirmed, would represent the second strategic setback for President Bashar al-Assad’s government this week.

The reported seizure of Al Jarrah airfield in Aleppo Province, which was corroborated by rebel video clips uploaded on the Internet, came a day after insurgent fighters announced that they had taken control of Syria’s largest hydroelectric dam, which supplies power to areas held by Mr. Assad’s security forces and by the insurgent Free Syrian Army and affiliated rebel groups. Whoever controls that dam, situated on the Euphrates River in northeast Raqqa Province, theoretically has the ability to deny electricity to significant areas held by the other side.

It was far from clear whether the insurgency’s claimed military gains signaled a bigger turn in the conflict, but some political analysts said they believed that the claims were credible and noteworthy. “Combined with capturing the dam, it’s another sign that Assad’s power is degrading but not yet finished,” said Andrew J. Tabler, a senior fellow in the Program on Arab Politics at the Washington Institute for Near East Policy.

There were unconfirmed reports late Tuesday of a mass defection by 40 members of Syria’s Fourth Armored Division and the Republican Guard, two of the best-trained and most loyal elements of Mr. Assad’s military, drawn largely from his Alawite minority that runs the country and is responsible for protecting him.