The convoy had been traveling from west to east. The ISIS fighters bombed by the coalition apparently had been coming to their rescue, on the same highway but from east to west. There were no reports from Syria that the convoy had reached its destination in Deir al-Zour Province in eastern Syria, the last major ISIS stronghold.
Lebanese officials confirmed that the Lebanese Army, in coordination with Hezbollah and the Syrian Army, arranged on Monday for 670 Islamic State fighters and their relatives to be taken nearly 300 miles in buses and ambulances from near the Lebanese-Syrian border to Abu Kamal, close to the border with Iraq.
Hassan Nasrallah, the leader of Hezbollah, said in a speech Monday night that the evacuees included 26 wounded fighters, 308 armed fighters and 331 civilians, presumably family members of the militants.
The Islamic State fighters had been surrounded by Lebanese and Syrian forces on both sides of that border. The group made the safe-passage deal to try to win the freedom of its fighters in exchange for turning over the bodies of nine Lebanese soldiers taken prisoner in 2014.
The bodies were given back to Lebanese forces between Sunday and Tuesday.
Colonel Dillon said the pact undermined efforts to fight the Islamic State in Syria. Iraq, normally an ally of the Syrian government, joined the American military in criticizing the decision to relocate the militants.
“The coalition, we are not party to this agreement between Lebanese Hezbollah and ISIS,” Colonel Dillon said. “Their claim of fighting terrorism rings hollow when they allow known terrorists to transit territory under their control. ISIS is a global threat, and relocating terrorists from one place to another is not a lasting solution.”
Colonel Dillon said airstrikes directly on the Islamic State convoy remained a possibility but as of late Wednesday had not been carried out because coalition officials were trying to verify whether civilians were intermingled in the group.
“We are monitoring these fighters in real time, we will take action where necessary, those would be absolutely lucrative targets,” he said.
“We’ve seen ISIS use protective sites like hospitals and mosques, seen them drive in ambulances,” the colonel said. “So if we do identify and find ISIS fighters who have weapons — we can discriminate between civilians and ISIS fighters — we will strike when we can. If we are able to do so, we will.”
The militants were loaded into 17 buses and 12 ambulances near Arsal, in northeastern Lebanon, on Monday morning, according to Hezbollah officials in Lebanon, and then taken to the Syrian city of Homs, to the north. Some of the buses were emblazoned with the name of a tour company, Happy Journeys.
In Homs on Tuesday, the Islamic State evacuees were transferred to buses and ambulances sent there by the Islamic State, according to the Syrian Observatory for Human Rights, a monitoring group based in London. They then began what would normally be a 10-hour journey to Abu Kamal.
By Wednesday, it was unclear how far the convoy had traveled, but Colonel Dillon said it was his understanding the coalition had stopped it before it reached militant-held territory in Deir al-Zour Province, and would not allow it to continue farther east.
Lebanese officials praised the safe-passage deal because it rid the country’s borders of Islamic State fighters and assuaged public concern over the fate of the nine missing servicemen. But others in the region were critical. The Iraqi prime minister, Haider al-Abadi, faulted Syria for moving the Islamic State fighters to its eastern frontier, along the border with Iraq.
“We fight the terrorists in Iraq,” he said in a speech on Tuesday. “We do not send them to Syria — we kill them in Iraq.”
Mr. Abadi called on the Syrian government to investigate the decision to relocate the Islamic State fighters.
“There should be full cooperation on the elimination of terrorism, not the evacuation of them from one place to the other,” he said.
The Lebanese government took pains to play down the cooperation between Hezbollah fighters and the Syrian Army on the Syrian side of the border, and of those two forces with its own army on the Lebanese side. It insisted that it was merely coincidental that both sides simultaneously began an offensive against the Islamic State over a week ago and that both sides declared a cease-fire in their fights against the militant group about the same time on Sunday.
Lebanon, which relies on American military aid for its armed forces, does not want to appear to be acting as an ally of the Syrian government. Hezbollah, which dominates Lebanese politics, makes no secret that it sends militias to fight in Syria and maintains a close alliance with Iran.
The American presidential envoy for the anti-Islamic State coalition, Brett H. McGurk, denounced the pact in Twitter posts.
“Irreconcilable ISIS terrorists should be killed on the battlefield, not bused across Syria to the Iraqi border without Iraq’s consent,” Mr. McGurk wrote. He added a thinly veiled threat: “Our coalition will help ensure that these terrorists can never enter Iraq or escape from what remains of their dwindling ‘caliphate.’”
A State Department spokesman in Washington, Edgar Vasquez, also noted that the United States had not been a party to the safe passage deal. “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,” he said.
Fighting in the Raqqa area of Syria, where the American-led coalition and allied Syrian groups are actively battling the Islamic State, has increased in intensity recently. The Syrian Observatory for Human Rights reported on Wednesday that 64 combatants had been killed in the last 24 hours and 150 in the last six days.
On Tuesday, the coalition reported, its warplanes carried out 56 airstrikes against Islamic State targets, including 51 airstrikes in the Raqqa area, four in Abu Kamal, where the convoy of Islamic State evacuees was headed, and one other.
On Wednesday, the Syrian government announced that its army had entered Deir al-Zour Province from the west to try to increase its encirclement of Islamic State positions. It was not clear where the Syrian Army’s positions were in relation to the trapped convoy.
It was also not clear whether the American airstrikes against the ISIS convoy took place over government-held territory in Syria, but the battlefield in Syria has become crowded and complicated.
Colonel Dillon confirmed earlier reports that American military forces in northern Syria were fired on by Turkish-backed anti-regime units near Manbij on Aug. 21 and returned fire against them. Colonel Dillon said the Americans had fired “less than 100 rounds” and took up defensive positions before the fighting was defused. He said an American mobile unit had clearly been flying the American flag and was deliberately targeted.
The American coalition supports Kurdish groups in northern Iraq, along with some Arab fighters in the Syrian Democratic Front, who are leading the attempt to retake the city of Raqqa from the Islamic State. To the west, insurgent fighters, including Arabs and Turkmen, aligned with Turkey, are fighting the government. Turkey has criticized its American allies for working with the Kurds, whom it views as allies of Kurdish separatists inside Turkey.
Mustafa Seijari, a rebel commander in the Turkish-backed Euphrates Shield, claimed that the shooting on the American forces had been accidental. “Our fighters received gunfire from S.D.F. positions nearby,” he said, referring to the Syrian Democratic Forces. “They returned fire not knowing of the presence of any coalition forces there.”
Correction: August 31, 2017
Because of a transcription error, an earlier version of this article incorrectly quoted the American military spokesman Col. Ryan Dillon as he described some of the parties to a safe-passage deal for Islamic State fighters. He referred to “Lebanese Hezbollah and ISIS,” not to Lebanon, Hezbollah and ISIS.