BEIRUT, Lebanon — As Syrian government forces on Saturday pushed into Yabrud, a town long held by rebels near the Lebanese border, opposition activists said that one of many fighters who fell in battle was a leader of the insurgent group the Nusra Front who had helped negotiate an unusual prisoner exchange last week.

Activists in and around Yabrud said the fighter, Abu Azzam al-Kuwaiti, had been killed in fierce combat in the area. Government forces, backed by the Lebanese militia Hezbollah, have spent weeks edging toward Yabrud, a critical transit point for militants, refugees and arms heading in and out of Lebanon.

Now the fall of Yabrud appears to be imminent.

Early in the uprising against President Bashar al-Assad, which began in March 2011, some Muslims and Christians in Yabrud worked together to protest against the government, and residents governed themselves in an orderly manner. The Syrian revolt, which began as a largely peaceful movement for political rights, has transformed into a bloody war that has killed an estimated 150,000 people. Activists said the Nusra Front commander, Abu Azzam, was a citizen of Kuwait. Foreign and local fighters in the area have worked together, sometimes with tensions, and some local residents have joined the front, which is said to be a terrorist group by the United States.

Abu Azzam helped negotiate the release of women and children held by the Syrian government, including members of his own family, in exchange for 13 nuns and three attendants held hostage by the front, a deal that incited a backlash from government supporters. A video of the exchange showed him reuniting with his children.