The Russian military requested US drones depart the area through the "de-confliction" line, as Russian-backed Syrian forces battle to recapture the ISIS-held city of Deir ez-Zor, located in eastern Syria.
Dillon said that at least 40 vehicles belonging to ISIS have attempted to rescue the stranded jihadists, including armored technical vehicles and a tank disguised as a truck.
"That still leaves half, or just under half, who are still out there or maybe in various stages of returning", he said, adding that officials have observed a "significant relocation" of European-born fighters to Libya.
US officials say about 300 ISIS fighters were part of the original group, accompanied by a similar number of civilians who were most likely family members. They rode on 17 buses bound for extremist-held areas farther east.
US led Coalition forces were using airstrikes to prevent the convoy from traveling further east.
However, the coalition has blocked the convoy from entering Daesh territory in east Syria, near the border with Iraq, by cratering roads and destroying bridges.
State-run SANA news reported that the Syrian army, backed by Syrian and Russian airstrikes, established control over a number of towns and hills southwest of Deir ez-Zor while forces in the city fortified their positions as they prepared to extend their territory.
Coalition spokesman Colonel Ryan Dillon told reporters at the Pentagon that the air raids were targeting militant engineers to cut down their abilities to use drones in the attacks.
US officials have disputed claims by Syrian opposition activists that dozens of the ISIS fighters and their families had still managed to cross into ISIS-held areas using civilian vehicles.
The convoy 'has not. and will not reach Iraq, ' Brett McGurk, special presidential envoy for the coalition, said Wednesday on Twitter. "We have struck individual ISIS fighters and fighters that leave in small groups to walk away, and as soon as they get far enough away from the buses - we have, and we will continue to strike ISIS fighters that venture far enough away where we can hit them without causing harm to the civilians that are a part of that convoy", he said.
"You could clearly tell they were going to fisticuffs", Dillon said. A delivery on Tuesday night ended with the fighters showing apparent signs of frustration at being pinned down as they attacked each other.
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