Airstrikes on Islamic State of Iraq and Syria (ISIS)-held villages in northern Syria killed at least 56 civilians on Tuesday as intense fighting was underway between the militants and U.S-backed fighters, Syrian opposition activists and the extremist group said.

Meanwhile, a U.S.-supported Syrian rebel group said Wednesday it will open an investigation into the beheading of a Syrian boy on spy accusations near Aleppo.

Residents in the area blamed the U.S.-led coalition for the strikes that targeted two villages, Tokhar and Hoshariyeh, which are controlled by ISIS, activists said. The villages are near the ISIS stronghold of Manbij, a town that members of the predominantly Kurdish U.S.-backed Syria Democratic Forces (SDF) have been trying to capture in a weeks-long offensive.

"We are aware of reports alleging civilian casualties in the area," Col. Christopher Garver, the chief spokesman for the U.S.-led war effort, told the Guardian newspaper, adding the coaltion is "looking into" the allegations to determine whether to launch a formal investigation. 

Conflicting reports of death toll 

The death toll from the airstrikes, which coincided with a wide ground offensive by the extremists against SDF fighters, ranged between 56 and 200. If it is confirmed that 200 people were killed, it would be the deadliest strike by the U.S.-led coalition since it began its military campaign against ISIS in Syria almost two years ago.

Conflicting numbers in the aftermath of attacks are not uncommon in Syria. There were also conflicting reports on where the civilians were killed, with some groups reporting that a school housing refugees was hit and others saying that people were struck as they fled the violence.

An international human rights group said the U.S.-led coalition, which has been carrying out airstrikes against ISIS in Syria since September 2014, must increase its efforts to prevent civilian deaths and investigate possible violations of international humanitarian law.

The Britain-based Syrian Observatory for Human Rights said at least 56 civilians, including 11 children, were killed in the strikes on the villages, which also wounded dozens. Another activist group, the Local Co-ordination Committees, said 90 people, mostly families, were killed.

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A Syria Democratic Forces fighter walks in the silos and mills of Manbij, a town that members of the predominantly Kurdish U.S.-backed SDF have been trying to capture in a weeks-long offensive. (Rodi Said/Reuters)

In a statement, the Syrian Foreign Ministry said that 120 civilians, mostly women, children and old people, had been killed in the airstrike on Tokhar village. It said the strike was conducted by French warplanes.

The ISIS-linked Aamaq news agency said that 160 civilians — mostly women and children — were killed in Tokhar alone, in a series of purportedly American airstrikes around dawn Tuesday, while a Facebook page for activists in the area put the death toll at more than 200.

The reports and the disparate casualty tolls could not be independently confirmed because the area is inaccessible to independent media. 

SDF says reports exaggerated 

A spokesman for the U.S.-backed SDF, Sherfan Darwish, said the numbers are exaggerated, adding that the coalition targeted gatherings of the ISIS group who were in the village, killing large numbers of militants. He added that the extremist group quickly buried its dead and claimed many civilians were killed.

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A woman, who fled the violence in Manbij city, arrives with her children to the southeastern rural area of Manbij on June 19. Airstrikes on villages near the ISIS stronghold claimed dozens of Syrian lives on Tuesday, rights groups say. (Rodi Said/Reuters)

Postings on a Facebook page show images of bodies, including those of children, being placed in a collective grave, purportedly in the village of Tokhar. One photograph shows a man carrying the lifeless body of a child covered with dust while another shows a child, partly covered by a blanket, lying in a grave.

The photographs appeared genuine and corresponded to other Associated Press reporting of the events depicted. Tuesday's casualties follow similar airstrikes on the ISIS-held town of Manbij on Monday, when at least 15 civilians were reportedly killed.

History of civilian casualties

The London-based rights group, Amnesty International, said in a statement Tuesday that since June, more than 100 civilians have been reportedly killed in suspected attacks by the U.S.-led coalition in the Manbij area.

"There must be a prompt, independent and transparent investigation to determine what happened, who was responsible, and how to avoid further needless loss of civilian life," said Magdalena Mughrabi, interim Deputy Director of the Middle East and North Africa Programme at Amnesty, following the airstrikes on Tokhar.

"Anyone responsible for violations of international humanitarian law must be brought to justice and victims and their families should receive full reparation," Mughrabi said.

'There must be a prompt, independent and transparent investigation to determine what happened, who was responsible, and how to avoid further needless loss of civilian life.' - Magdalena Mughrabi, Amnesty International 

The statement said Amnesty International has reviewed available information on dozens of suspected coalition airstrikes and found that in the majority of cases in which civilian casualties have been credibly reported, the coalition has dismissed the claims.

Syrian rebels accused of beheading boy

Meanwhile, the Nour el-Din al-Zinki group,  a relatively moderate group that fights the Syrian government and ISIS, said it will open an investigation into the beheading of a Syrian boy, calling it an "individual mistake" that does not represent the overall policies of the group.

The boy was identified by activists on social media as Mahmoud Issa, a 12-year-old Palestinian. The Syrian government also denounced the "repulsive crime" against an innocent child in a statement to the United Nations, saying the boy was a Palestinian from the Palestinian refugee camp of Handarat on the edge of Aleppo.

The incident allegedly occurred in Handarat, near Aleppo, on Tuesday. A young boy is seen on the back of a pickup truck surrounded by armed men who accuse him of being a spy and a member of the Quds Brigade, a pro-government Palestinian militia. He is then beheaded with a knife by one of them.

The Quds Brigade, in a statement posted on its Facebook page, strongly denied that Issa was a fighter, adding that the boy was ill. It said he was killed by a "terrorist" who lost his brother in battles near Handarat apparently as revenge.

Nour el-Din al-Zinki said everyone involved in the violation was arrested and are being investigated. The group, which operates mainly in Aleppo, has received support in the past from the United States.

The U.S. State Department said on Tuesday that it was investigating the incident.

With files from CBC News