BEIRUT, Lebanon — A loyalist of Osama bin Laden who trained fighters to battle American troops in Iraq and became a commander of Al Qaeda’s affiliate in Syria was killed there in the last week along with three fellow leaders, according to Syrian insurgents and a monitoring organization.

Reports differed on exactly when and how the commander, Samir Hijazi, and the other leaders of the affiliate, the Nusra Front, were killed. But the deaths of so many top figures, if confirmed, would signal a sharp blow to the Nusra Front, one of the strongest insurgent factions fighting the Syrian government.

The reports led other rebels in the four-year-old Syrian conflict to speculate that the group had been infiltrated by enemies.

The Nusra Front was the first major jihadist group to make its presence known in Syria, although it has since been eclipsed by the Islamic State, which has declared a caliphate on territory it controls in Iraq and Syria and has shocked the world with videos showing the beheading and incineration of captives.