Buses carrying hundreds of Daesh militants and their families arrived in eastern Syria on Tuesday following a negotiated evacuation from the Lebanon-Syria border, where the U.S.-backed Lebanese army deployed for the first time in years.

The evacuation agreement, the first such publicized deal concluded with the terrorist group, angered many Iraqis, who accused Syria and Hezbollah of dumping the militants on the Iraqi border rather than eradicating them.

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. The deal also provoked controversy in Lebanon, as some have voiced opposition to negotiations with the militants.

Iraqi Prime Minister Haider al-Abadi said his government has sought an explanation from Syria.

"Honestly speaking, we are unhappy and consider it incorrect," al-Abadi told reporters. "Transferring terrorists from Qalamoun (along the Lebanese-Syrian border) to the Iraqi-Syrian border is worrying and an insult to the (Iraqi) people."

"There must be no chance for Daesh to breath," he added.

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

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

Shiite-majority Iraq has been largely supportive of Syria and the Iran-backed Hezbollah in their battle against Daesh, a Sunni extremist group and shared enemy. But many Iraqis expressed anger about the evacuation deal on social media.

"Who's going to pay for the delivery sent by Hassan Nasrallah to us?" wrote well-known writer Saleh al-Hamdani on his Facebook page, referring to Hezbollah's leader. "Will (state-sanctioned militias) or the Federal Police or Counter Terrorism Forces pay for it?"

Baghdad-based analyst Hisham al-Hashimi wrote on Facebook that Iraq's "selfish ally preferred to throw Daesh danger from Lebanon to Iraq, while Iraqis demolished the second-largest city (Mosul) in order not to enable Daesh militants to flee (to Syria) and damage the neighbor."

The Lebanese government and Hezbollah have both defended the deal that allowed Daesh safe passage to the eastern province of Deir el-Zour, a Daesh stronghold, saying it was the only way Lebanon could uncover the fate of its captured soldiers and recover their remains.

But many in Lebanon were opposed to the deal, which they said allowed the killers of Lebanese soldiers to get off scot-free.

"Shame on a nation whose soldiers return in coffins, while the criminals leave in air-conditioned buses," many posted on social media.

The departure of Daesh marked the end of a militant presence along the Syrian-Lebanese border dating back to the early days of the Syrian uprising. The Lebanese army was able to assume full control of the border.

Syrian state-run news agency SANA said the buses carrying the militants arrived Tuesday to a handover point in the town of Hamimiyah in the eastern Deir el-Zour province. They are expected to continue from there to the town of Boukamal, near the Iraqi border.