WASHINGTON — A senior official of Al Qaeda was killed on Sunday in Syria by a United States drone strike, an American official said on Wednesday.

The terrorist leader, Abu al-Khayr al-Masri, 59, was the second-ranking official after Ayman al-Zawahri and was a son-in-law to Al Qaeda’s founder, Osama bin Laden.

Jihadist social media carried reports of the attack on Sunday, including photographs of the vehicle purportedly hit by the drone. The strike was said to have taken place in Idlib Province in northwest Syria, where the Pentagon has stepped up airstrikes against top Qaeda operatives in the last two months, killing several important figures.

Charles Lister, a senior fellow at the Middle East Institute and an expert on the war in Syria, said in an email that the death of Mr. Masri was the most significant blow to Al Qaeda’s global network since the killing of Nasir al-Wuhayshi, Al Qaeda’s No. 2 official at the time, in a drone strike in Yemen in June 2015.