American aircraft have been picking off Islamic State fighters who stray too far from their bus convoy stranded in the Syrian desert, including those who may wander too far in order to relieve themselves.

Eleven buses filled with Islamic State fighters and their families have been stalled in the desert for the past 72 hours, occasionally being resupplied by trucks coming from Syrian regime-controlled territory, U.S. defense officials say. American aircraft buzzing overhead are blocking them from reaching an Islamic State stronghold in Deir Ezzor, and so far they appear unwilling to head back to Syrian-controlled territory.

But any attempts at resupply coming from the east — areas under Islamic State control — are being taken out by U.S. aircraft following the convoy.

At least 40 vehicles have been hit by U.S. airstrikes in recent days, and American planes have been picking off Islamic State fighters by ones and twos, Col. Ryan Dillon, a spokesman for the U.S. military command in Baghdad, told Foreign Policy.

During the most recent round of strikes Tuesday evening, U.S. aircraft struck fighters who got too far away from the buses. “Whether it’s to evade by foot or to relieve themselves, if they make it far enough out there for us to strike, then we will,” Dillon said.

Just after a resupply run Tuesday evening, American surveillance also picked up a rolling fistfight among several of the stranded fighters, Dillon said, indicating that after a week in the desert, tempers are flaring.

The trek by somewhere between 300 and 500 Islamic State fighters and their families began eight days ago, after the extremists surrendered to the Syrian regime and its Hezbollah backers near the Lebanese border. Part of the truce involved putting the group, which included women and children, on 17 buses and sending them across the country to the Islamic State bastion in far-eastern Syria along the Iraqi border.

That’s when the deal fell apart.

American commanders objected to the truce and then bombed a roadway and disabled a bridge in the convoy’s path. By this weekend, six of the buses had turned around and headed back to regime-controlled territory, leaving 11 sitting in the desert northwest of the city of Abu Kamal, near the Iraqi border.

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Brett McGurk, the U.S. envoy to the international coalition fighting the Islamic State, tweeted last week that the United States “will not allow this terrorist convoy to further approach #Iraq’s borders” and that militants “should be killed on the battlefield, not bused across #Syria to the Iraqi border without #Iraq’s consent.”

The U.S. coalition also delivered a message to Damascus through Russian intermediaries last week, warning against trying to drive Islamic State fighters east, saying the United States would not allow it.

In a statement on Sunday, Lt. Gen. Stephen Townsend, the commander of U.S. and allied forces in Iraq and Syria, said, “The Syrian regime is letting women and children suffer in the desert. This situation is completely on them.”

The U.S. Defense Department has said a hotline between the American and Russian militaries operating in Syria remains open and that the two sides speak on a daily basis, despite Moscow’s objections to the United States stopping the convoy in the desert.

Photo credit: LOUAI BESHARA/AFP/Getty Images