A day after an American raid killed Islamic State leader Abu Bakr al-Baghdadi, a separate attack killed the man who was considered his likely successor, according to the leader of a Syrian Kurdish militia and a Syrian activist.

The spokesman, known by his nom de guerre Abu Hassan al-Muhajir, was being smuggled across northern Syria in the back of an oil tanker truck when it was hit by what witnesses said they believed to be an American airstrike, according to Hussein Nasser, an activist who said he had spoken to people at the scene.

Mazlum Abdi, the head of a Kurdish-led militia that fought the Islamic State with the United States, wrote on Twitter that Mr. al-Muhajir had been killed on Sunday in an operation coordinated between his forces and the United States.