Update 5.12pm:The US-led coalition says it has struck an Islamic State convoy coming from militant-held territory in Syria to meet IS evacuees being transferred under a controversial deal.
US officials have criticised the transfer of hundreds of militants and civilians who are heading for an IS-held area near the Iraqi border, saying the extremists should be killed on the battlefield.
Col Ryan Dillon said the airstrike hit vehicles identified as belonging to IS that were travelling from IS-held territory toward the convoy.
Coalition officials say they are not bound by the evacuation agreement brokered by Lebanon's Hezbollah militant group, and may strike the evacuees themselves.
An earlier coalition airstrike destroyed a small bridge and left craters in a road, hindering the movement of the evacuees.
There are about 300 militants and almost as many family members on buses being evacuated under the deal, which ended the IS presence along the Lebanon-Syria border.
Earlier:A convoy of Islamic State fighters evacuated from the Lebanon-Syria border under a controversial agreement is being closely monitored, officials say.
Col Ryan Dillan, a spokesman for the US-led alliance against the militants, said "we are monitoring their location in real time".
He added that the coalition "will not rule out strikes against IS fighters being moved".
He said the coalition has already bombed a small bridge to obstruct the convoy.
Syrian opposition activists say the convoy, which left the Lebanon-Syria border on Tuesday, is still in government-controlled territory in eastern Syria.
The IS militants were allowed to evacuate in buses following a Hezbollah-negotiated deal that will see them to go to IS-held territory near the Iraqi border.
Col Dillan said: "We are not party to any agreements that were made by the Lebanese Hezbollah and Isis or the (Syrian) regime."