A U.S. official has blasted a deal that led to the evacuation of hundreds of Islamic State group fighters and civilians from the Lebanon-Syria border to areas close to Iraq, saying the extremists should be killed on the battlefield.

The evacuation agreement, the first such publicized deal, had already angered many Iraqis who accused Syria and Lebanon's Hezbollah of dumping the militants on the Iraqi border rather than eradicating them.

The top U.S. envoy for the international coalition against ISIS, Brett McGurk, tweeted on Wednesday that it is "irreconcilable" that ISIS "terrorists should be killed on the battlefield, not bused across #Syria to the Iraqi border without #Iraq's consent."

McGurk added that the anti-ISIS coalition will help ensure that "these terrorists can never" enter Iraq.