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BEIRUT -- U.S. airstrikes blocked the advance of an Islamic State convoy carrying militants toward Iraq on Wednesday, derailing a Hezbollah-negotiated deal that removed the extremists from the Lebanon-Syria border, where they have been for years.

The airstrikes came amid U.S. criticism of the deal, reflecting a growing outrage within the Trump administration over the decision to give the militants safe passage from the battlefield instead of killing them, and Iran-backed Hezbollah's leading role in it.

The developments also were an embarrassment for the U.S.-backed Lebanese military, which agreed to the deal and had declared victory over the militants.

U.S. officials said the airstrikes to disrupt the fleeing militants were intended to send a strong signal that the deal, while helping to clear IS from the border, undermined a broader U.S.-led strategy for defeating the group in Syria and Iraq.

More than 48 hours after they left the Syria-Lebanon border for eastern Syria, the buses carrying 300 militants and almost as many of their relatives were stuck in a desert area on the outskirts of the largely IS-held Deir el-Zour province near the frontier with Iraq.

It is not clear how the standoff will be resolved. Syrian activists say alternate routes are being considered to bring the militants to Boukamal, an IS-controlled town on the Iraqi border, according to the agreement.

But officials of the U.S.-led coalition said they will continue to monitor the convoy and aren't ruling out more airstrikes.

"Irreconcilable (IS) terrorists should be killed on the battlefield, not bused across Syria to the Iraqi border without Iraq's consent," according to a tweet from Brett McGurk, the top U.S. envoy for the anti-Islamic State coalition.

The evacuation deal followed separate but simultaneous weeklong offensives by the Lebanese army on one side of the border and by the Syrian government and Hezbollah on the other. Hezbollah has thousands of fighters shoring up the forces of President Bashar Assad.

Iraq also reacted angrily to the evacuation, with its president saying that moving the militants to the Iraqi-Syrian border was an "insult."

Later Wednesday, the coalition said its warplanes struck a small bridge and cratered a road to hinder the convoy without targeting the evacuees. Airstrikes also hit a separate group of IS militants travelling to meet the convoy, according to Col. Ryan Dillon, a coalition spokesman.

Responding to the criticism but not addressing the airstrikes, Hezbollah leader Hassan Nasrallah said in a statement that negotiating with the militants was the "only way" to resolve the "humanitarian and national" issue of finding the remains of nine Lebanese soldiers that the militants kidnapped in 2014.

Hezbollah is a significant player in Lebanon with government ministers and lawmakers, while the role of its fighters also has been growing in Syria.

The Syrian government, backed by Russian air power and Iranian-organized militias including Hezbollah, has focused its military campaign in recent weeks on Deir el-Zour, where government troops have been besieged for years in the provincial capital.

Dillon criticized Moscow and Damascus for allowing the buses of militants to travel through territory they control.

"To say they are serious about defeating IS looks suspect right now," Dillon told The Associated Press.

Nasrallah, in his third statement on the issue in a week, said his fighters are battling alongside the Syrian troops to oust IS militants from the area where the others are headed.

"We transported those defeated militants from one front we fight in to another front we also fight in," he said.

Unease has been growing in the Trump administration about Lebanon's campaign against IS on its border because of the apparent co-ordination between Lebanon's military and Hezbollah. Lebanon maintains it's an independent operation.

In parts of Syria that have been freed from IS control, there is a growing influence by Hezbollah, and that has raised concerns about a stronger Iranian hand in Syria and the potential for Hezbollah to use the territory as a base to attack Israel.

But Lebanon's move to halt the fight after IS was squeezed into a small sliver on the border and then let the militants flee has further alarmed Washington. President Donald Trump has placed a particular emphasis on killing IS members while faulting former President Barack Obama for being too willing to let the militants regroup elsewhere.

The U.S. said it wasn't consulted as part of Lebanon's deal with IS, Hezbollah and the Syrian government, and it wouldn't have agreed to it if asked.

"We have obvious concerns, however, for any action that provides ISIS capabilities to shift its forces and thus put more civilians in harm's way," said Edgar Vasquez, a regional spokesman for the U.S. State Department.

Still, U.S. officials pushed back on speculation in Lebanon that the U.S. might cut aid or other help to Lebanon's armed forces, saying no such halt was being considered. U.S. officials said Lebanon's military, which enjoys broad confidence from the public, was the only institution stable and credible enough to hold together the country's delicate political situation. The U.S. officials spoke on condition of anonymity because they weren't authorized to talk to reporters.

Lebanon's military has defended the agreement.

Army commander Gen. Joseph Aoun said that as the offensive against IS was underway, the Lebanese mediator called him to say the extremists accepted a cease-fire in return for information about the fate of the missing soldiers.

"I had one of two choices. Either to go on with the battle and not know the fate of the soldiers, or give in and know the fate of the soldiers," he said. Remains of several people have been uncovered in the border area, and DNA tests are underway to determine whether they belong to the missing soldiers. Lebanese officials say they are almost sure they are.

Officials said there could be a silver lining for the coalition: While the IS militants had been holed up in a mountainous region of the Lebanon-Syria border where they were difficult to target, the evacuation deal means they will relocate to desert areas where coalition forces can strike them more easily.

Lederman reported from Washington. Associated Press writers Zeina Karam and Sarah El Deeb contributed from Beirut.