BEIRUT — For four days now, an Islamic State convoy with more than 600 people has been stuck in the middle of the harsh Syrian desert, blocked by US airstrikes, as a deal to allow it safe passage across Syria has collapsed.

Syrian opposition activists said Saturday that parts of the 17-bus convoy had pushed into areas of Syria controlled by the militants, despite US threats to bomb the group fleeing from the Lebanon-Syria border, the Associated Press reported.

The US-led military coalition has used airstrikes to prevent the convoy from reaching its destination in the Islamic State stronghold of Deir el-Zour province, and vowed to continue doing so, criticizing the Lebanese and Syrians for trying to relocate terrorists.

The convoy includes 308 lightly armed Islamic State fighters with 330 of their relatives in the buses. A dozen ambulances carry 26 wounded fighters.

By relocating them, Syria and Lebanon removed any Islamic State presence on the western border and concentrated it in the east, where Iraqi and US forces would have to deal with them.

Now it appears that even if the convoy does reach Islamic State territory, the fighters’ own comrades may not be pleased to see them. In addition, the Syrian regime’s allies appear to be extracting more concessions to allow the convoy to keep moving.

On Thursday, the fighters and their relatives were on the move again, according to Reuters. Quoting a Syrian military source, Reuters said the convoy had retraced its route from the southeastern border near Humaimah, in a 100-mile western loop and then north in an effort to reach Deir el-Zour through the Syrian town of Sukhna.

US military officials said that coalition warplanes would make sure that did not happen.

“They’re on the move. They’re trying a different route; we’re watching all the way through,’’ the US coalition spokesman, Colonel Ryan Dillon, said Thursday by telephone from Baghdad.

Dillon said the coalition flew airstrikes Wednesday against other Islamic State units that had tried to reach the convoy from inside Deir el-Zour.

While US soldiers and their Syrian allies are not actively fighting on the ground in Deir el-Zour, coalition airstrikes against the Islamic State take place daily there, and the area is expected to be the next major battlefield after Raqqa, the Islamic State capital, is subdued.

So far the coalition has not bombed the convoy directly because it is carrying civilians.

On Wednesday, coalition airstrikes cratered the highway in front of the convoy and destroyed a bridge in the town of Humaimah in Homs province, close to the Deir el-Zour provincial border, according to reports from the Syrian Observatory for Human Rights in London. This prevented the convoy from moving ahead.

On Monday morning, the Islamic State fighters had agreed to pull out of a 27-square-mile enclave they held on the border between Lebanon and Syria, in a deal they cut with both governments, and with Hezbollah militiamen from Lebanon who are fighting alongside the Syrians.

Under the deal, Lebanon received the remains of nine soldiers taken prisoner by the Islamic State in 2014; the Islamic State fighters received free passage to Deir el-Zour, 300 miles to the east.

Rita Katz, head of the SITE Intelligence Group, which monitors the Islamic State and other extremists online, said that Islamic State supporters refused to believe the group had made the deal to evacuate.

"Some users on a pro-ISIS chat group called reports of the event ‘fake news,’ asking, ‘Since when is news from Hezbollah authentic?’ ’’ she said.

Al Akhbar, a newspaper in Lebanon that generally supports Hezbollah, reported that Islamic State forces in eastern Syria were upset with the convoy’s occupants for abandoning their border enclave.

They did so after an apparently coordinated weeklong offensive against them by the Lebanese army, Hezbollah, and the Syrian army, which had surrounded them. Once the convoy was blocked in Humaimah on Wednesday, it apparently had to negotiate permission from the Syrian regime and its Hezbollah allies to try another route.

Hezbollah demanded that the Islamic State militants turn over the body of an Iranian fighter whom they had killed, Reuters reported, quoting the Syrian military source. It was not clear if that was a new deal, or part of the original deal Sunday.