The US-led coalition has killed dozens of jihadists linked to a convoy of Islamic State buses stranded in the middle of the Syrian desert, a US military official said Thursday.

The convoy, which initially consisted of 17 vehicles, has been stalled in the Deir Ezzor region since August 29.

US officials say about 300 IS jihadists were initially aboard, along with a similar number of civilians, likely family members.

The fighters had been headed from Lebanon to the Iraq border under an evacuation deal negotiated between IS and the powerful Lebanese Shiite movement Hezbollah, which has intervened in the war in neighboring Syria to prop up the Damascus government.

The United States was not party to the deal and had blocked the convoy just short of the border by bombing the road and a bridge leading from the Syrian town of Hmaymah to the IS-held town of Albukamal further east.

Colonel Ryan Dillon, a US military spokesman, said the coalition has not targeted the convoy itself and was permitting food and supplies to reach the stranded vehicles, but he noted about 85 IS fighters either from the convoy or heading by vehicle to link up with it had been picked off.

"We have struck individual ISIS fighters, and fighters that leave in small groups to walk away," Dillon told Pentagon reporters in a phone briefing from Baghdad.

"As soon as they get far enough away from the buses, we have and will continue to strike ISIS fighters ... where we can hit them without causing harm to the civilians that are part of that convoy."

The 17-vehicle convoy split in two last week, with six buses heading west toward the Palmyra region, which is under Syrian regime control.

"Those buses drove further into western Syria, we just made the decision to stop monitoring it as they drew further into the interior," Dillon said. 

The coalition has offered to try to bring the situation to a head by contacting Russia and offering a proposal that would allow the civilians to escape.

Dillon said that proposal had not gained any traction, leaving open the question of the fate of the IS fighters and the civilians.

"We don't see it as our issue," he said.