WASHINGTON — American Special Operations forces in eastern Syria killed a top Islamic State commander this week, Pentagon officials said Friday, part of a monthslong campaign the Obama administration boasts is eviscerating the Islamic State even as the group continues to demonstrate the power to sow violence in Western Europe.

The American forces originally hoped to capture the commander, Abd al-Rahman Mustafa al-Qaduli, and were following his vehicle in at least two helicopters, according to a senior military official who requested anonymity. But their plan to land Special Operations fighters, seize Mr. Qaduli and return him to the helicopter changed for unknown reasons, and they fired on the vehicle instead, killing him.

Defense Secretary Ashton B. Carter announced Mr. Qaduli’s death, describing him as the group’s top financier, but he offered few details of how he had been killed. Mr. Carter said the military effort against the Islamic State was gaining momentum, repeating a claim he has made for the past three months. He said the group was steadily being drained of leaders, soldiers and money, and was losing its grip on the parts of Iraq and Syria it had controlled.

Image An undated photograph released by the United States State Department of Abd al-Rahman Mustafa al-Qaduli, an Islamic State leader.



Credit... State Department/European Pressphoto Agency

The announcement came just three days after the Islamic State killed 31 people in a series of explosions in Brussels, showing in gruesome fashion that its abilities to conduct large-scale terrorist attacks are hardly diminished. But the week’s events could offer a glimpse of what is to come in the next year as the Islamic State and the United States-led coalition to defeat it engage in a series of punches and counterpunches, with each side claiming the upper hand.