Officials from the United States and Kurdistan Region presidencies have separately spoken out against reports of hundreds of ISIS militants being transferred from the Syria-Lebanon border area to the northeastern Syrian-Iraqi border.
The transfer under Syrian military escort was agreed under a ceasefire deal.
It is not clear how the standoff will be resolved.
Hizballah said "hundreds of IS fighters" had left the border area of eastern Lebanon, but this was not confirmed by the Lebanese army.
The coalition is reluctant to strike the actual convoy of evacuees because the fighters have wives and children traveling with them, said a USA official, who wasn't authorized to discuss military details and requested anonymity.
"Irreconcilable ISIS terrorists should be killed on the battlefield, not bused across Syria to the Iraqi border without Iraq's consent", tweeted US Special Envoy for Global Coalition to Defeat ISIS, Brett McGurk, who also spoke for the coalition's efforts under the Obama administration.More news: These 3 HTC Devices are Getting Android Oreo
The evacuation comes a week into a Lebanese army campaign against the group on the Lebanese side of the border, coinciding with a joint Syrian army and Hizballah offensive over the border. Hezbollah has thousands of fighters shoring up the forces of President Bashar Assad.
The U.S. coalition strikes on Wednesday to block the convoy moving into Islamic State territory took place east of Humeima in southeast Syria, near the edge of land held by the Syrian government, coalition spokesman Ryan Dillon told Reuters.
He also said the coalition would help ensure they "can never enter Iraq".
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.
But the plans of Israel which he described as the "enterprise of occupation and hegemony" had failed and "the ones who are crying today about the surrender of ISIS in Syria and Iraq are Netanyahu and the senior Israeli officials".
The end game is "to make it tougher for the Coalition to capture Deir al-Zor province from ISIL, by relocating their fighters there" the defence expert said.
Some 600 militants were allowed to leave as part of a deal, negotiated by Hezbollah, in exchange for identifying the location of the remains of Lebanese soldiers captured by Daesh in 2014 and later killed.More news: More Voters Trust Media Over Trump
"We will be seriously observing this and looking into this".
Almost two dozen buses and 11 ambulances carried the militants and their families Monday from the area straddling the Syria-Lebanon border toward the Islamic State-held town of Boukamal in eastern Syria. 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.
Nowruz Ahmed sits on the military council of the USA -backed Syrian Democratic Forces (SDF) and as one of a small number of members of its Raqqa general command is one of the most senior commanders in the offensive.
A US State Department spokesperson told PRI that "the Lebanese Armed Forces has been successfully defending Lebanon's borders and fighting ISIS and other extremists on the front lines". "We do not send them to Syria", he said in a speech. "So they had no choice but to accept our conditions", he said.
Lebanon's military has defended the agreement. "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", Aoun said.
The coalition nevertheless said in a statement that it is not bound by the evacuation agreement.
Once the last of the militants depart, the border area will be free of insurgents for the first time since the early days of the Syrian civil war, which began in 2011.More news: Ram Rahim Singh rape trial: CRPF, RAF to be deployed during hearing