The group has since lost much ground to separate counteroffensives by Syrian and Iraqi forces as well as American-led operations, and its presence is now confined to a few areas. The group is estimated to control no more than 3 percent of Syria’s territory.

Last December, the Iraqi government declared victory over the Islamic State, but the Iraqi military has continued regular operations in desert areas along the porous border with Syria.

Reports about the death of Mr. Baghdadi, the group’s leader, have circulated on several occasions, but he is now living in Syrian territory by the Iraqi border, an Iraqi intelligence official said in May. Mr. Baghdadi, originally from Iraq and formerly known as Ibrahim Awad Ibrahim al-Badri, was said to be moving around with only a small group of followers.

The United States is offering a $25 million reward for his capture.

In March, five senior Islamic State officials were captured, including a top aide to Mr. Baghdadi, in an operation carried out by Iraqi and American intelligence, Iraqi officials said at the time.

In September, in the last voice message attributed to Mr. Baghdadi, he called on his fighters in Syria and Iraq to “resist” their enemies and urged followers across the world to wage attacks against the West.