BEIRUT, Lebanon – A mortar shell fired from inside a besieged Palestinian refugee camp struck a Red Cross vehicle on Monday, killing two of the aid agency’s workers and critically injuring a third, Lebanon’s state-run news agency and security officials said.

The National News Agency said the two local aid workers were killed near the northern edge of Nahr el-Bared camp by shelling from Fatah Islam militants holed up inside. A security official said later in the day that troops had destroyed the residence and headquarters of the militant group’s leader.

Other security officials said the workers were killed when their vehicle was struck by what appears to be a mortar shell fired from inside the camp. A third Red Cross worker was seriously wounded.

The security officials, who spoke on condition of anonymity because they were not allowed to give official statements to the press, said there were army casualties as well.

A Muslim sheik trying to mediate an end to the fighting between the Lebanese army and the militants was wounded as he left the camp in a Palestinian Red Crescent vehicle.

One security official said troops leveled the residence and headquarters of Fatah Islam leader Shaker Youssef al-Absi. It was not clear if al-Absi was there during the attack.

The NNA said the army seized “important documents” from al-Absi’s headquarters. It also said troops repelled an infiltration attempt by Fatah Islam members, killing seven of them. The report could not be immediately verified.

The leading An-Nahar newspaper reported Monday that “the Nahr el-Bared battle is headed toward a big escalation,” saying the Lebanese military had brought in new reinforcements, including more effective artillery and additional naval forces, while pro-Syrian Palestinian factions had joined Fatah Islam militants in their fight.

On Saturday, the Lebanese army – using tanks and artillery – launched a fresh offensive.

“The army continues to expand its control in and outside the camp with the aim of tightening the noose around the Fatah Islam gunmen,” a senior military official told The Associated Press Monday, speaking on condition of anonymity because he was not authorized to give official statements.

One Lebanese soldier was killed in the fighting Monday, raising the number of soldiers killed in the Nahr el-Bared fighting since it began on May 20 to 58, the military official said. Two soldiers were killed in last week’s clashes with Jund al-Sham militants in the Palestinian refugee camp of Ein el-Hilweh in southern Lebanon.

More than 130 people in total, including at least 60 Fatah Islam militants, 20 civilians and the two Red Cross workers, have been reported killed in the fighting in northern Lebanon – the worst internal violence to engulf the trouble-plagued country since the 1975-90 civil war. The Red Cross did not immediately return calls seeking comment.