BEIRUT (AP) — Gunmen opened fire at the convoy of a joint humanitarian assessment mission in eastern Syria, the United Nations said Saturday, wounding a local driver in one of the country's most remote areas where fighting against Islamic State militants has displaced thousands.

The attack on Thursday complicated an already dire situation, where a lack of security in the area has made delivering assistance to at least 6,000 displaced people unsustainable.

Fadwa Baroud, of the Office of the U.N. resident coordinator, said the driver is recovering from gunshot wounds. Two gunmen on a motorcycle opened fire at the clearly marked convoy about 50 kilometers (30 miles) from where the U.S.-led coalition and local Syrian fighters are battling militants in their last pocket, in eastern Deir el-Zour province.

The convoy was dispatched to assess dire humanitarian conditions due to the escalation in fighting.

Baroud said 6,000 people have been displaced since October and an estimated 7,000-10,000 others remain in the IS-held Hajin enclave.

"We have been able to provide assistance to the 6,000 IDPs however, insecurity in the area limits our ability to sustain the response," she said.

The last pocket of IS in eastern Syria lies along the border with Iraq and on the eastern banks of the Euphrates River. The U.S.-led coalition and its local allies, the Kurdish-led Syrian Democratic Forces, launched an offensive in September to dislodge the militants from the area.

But the offensive has dragged on, and activists have reported a large number of civilian casualties as militants prevent them from leaving the area.

The Britain-based Syrian Observatory for Human Rights said dozens of families managed to escape Friday, using waterways across the river which were formed after heavy rains in recent days. The militants have previously executed people for attempting, or aided others, to escape.

The violence, including clashes between the SDF and the militants as well as coalition airstrikes, have left at least 51 civilians, including 19 children, killed in one week, the Observatory said. Airstrikes hit a makeshift prison and hospital in Shafaa and Kishmeh, two IS-controlled villages, late Tuesday, killing 38 including fighters and prisoners, the Observatory reported.

Baroud said the U.N. was informed of the reports. 

"The United Nations continues to receive disturbing reports of hostilities impacting the civilian population in south-east (Deir el-Zour). We continue to call upon all parties to protect civilians and to facilitate humanitarian access to the affected population," she said.

A spokesman for the U.S.-led Coalition, Col. Sean Ryan, said the coalition has targeted and destroyed what he called IS "command and control facilities" in Kishmeh, killing four militants.

"We conducted the strikes on (IS) fighters after an in-depth monitoring of the building to reduce the risk to civilians.  We reject any (IS) propaganda claims on striking an (IS) prison," Ryan said in an email to The Associated Press Friday.