Save the Children on August 28 also said a bombing campaign has increased the "nightmares" for young people in the self-declared IS capital, which is under siege by US -backed Syrian Arab and Kurdish fighters.
Iraqis on Tuesday denounced a deal allowing Islamic State group fighters to evacuate a Syrian-Lebanese frontier region towards the Iraqi border.
He said Iraq was battling the fighters, not sending them to Syria.
"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", said Edgar Vasquez, a regional spokesman for the U.S. State Department.
The agreement saw the air-conditioned buses move the ISIS members from Qalamoun on the Lebanese border to the town of Abu Kamal in the eastern Syrian province of Deir Ezzor.
The main security body of the Iraqi Kurdistan region also condemned the deal, questioning the motivation of the Syrian regime and its key Lebanese ally for dealing with the jihadi group responsible for capturing swathes of Syrian and Iraqi territory since 2014 and the murder of thousands.
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Saad al-Hadithi, a spokesman for the Iraqi government, told the Associated Press: "Any deals or understandings between the warring parties inside Syria or in the region must take into consideration the security of Iraq and not to lead to anything that poses any threat to our national security".
"It had no choice but to accept our conditions", he added, emphasizing Hezbollah's role in revealing the fate of the slaughtered Lebanese soldiers and securing their bodies.
Isis has lost swathes of territory since 2015 in both Syria and Iraq, including the Iraqi city of Mosul, which was eventually wrestled from the terrorists in July this year.
Lebanon has defended the agreement, in which the militants are said to have revealed the location of the remains of nine Lebanese soldiers who were captured in 2014.
The militants and their families left the border area on Sunday under a ceasefire deal.
The battle to oust Islamic State from its stronghold in the Syrian city of Raqqa should end within two months, a top-ranking Kurdish commander told Reuters, but said she expects the fighting to intensify.
The militants were given safe passage to the Syrian province of Deir al-Zor. "Their souls are my responsibility", he said.
"The negotiations have laid bare the close links between [Islamic State], the Iranian-backed Hezbollah and the Assad regime as well as the complicity of the three parties in the spread of terrorism in Syria and Lebanon", Syria's main opposition group said in a statement.