An image posted Tuesday on a militant website purports to show fighters from the Islamic State firing weapons toward troops and pro-government gunmen in Deir al-Zour province in Syria. (AP)

Scores of civilians died Tuesday in airstrikes allegedly carried out by U.S. warplanes on a Syrian village in an area that is the target of a U.S.-backed offensive, according to human rights groups and opposition activists from the area.

The U.S. military said it is investigating claims that American airstrikes were responsible for the deaths of as many as 160 civilians early Tuesday in Tokhar, 10 miles from the northern town of Manbij.

A military statement said investigations are underway into whether the site where the victims died matches any of the locations bombed at the time by American warplanes.

“If the information supporting this allegation is determined to be credible, we will then determine the next appropriate step,” the statement said. The U.S. military takes “all measures during the targeting process to avoid or minimize civilian casualties,” it added.

The alleged strikes coincided with intense fighting in the Manbij area as a U.S.-backed coalition of Kurdish and Arab fighters called the Syrian Democratic Forces presses ahead with a seven-week-old offensive to capture Manbij. Earlier Tuesday, the U.S. military announced that the SDF had secured control of the Islamic State headquarters in the town. A separate statement said American warplanes had conducted 18 strikes in the previous 24 hours around Manbij.

Other warplanes involved in the widespread bombing of Syria, including the Russian and Syrian air forces, occasionally stray into areas where the U.S. military is operating.

People from the Manbij area said, however, that U.S.-led coalition warplanes are the only ones that have been bombing in the area since the offensive was launched June 1.

If Tuesday’s reports are confirmed, the tragedy could undermine local support for the U.S.-backed effort to free the strategically important town from militant control, said Jassem al-Sayed, a member of the local council that governed Manbij on behalf of the Syrian opposition before the Islamic State’s takeover in January 2014.

“People are now full of hatred for the SDF. We thought they were coming to finish ISIS, but it seems they are finishing us first,” he said, speaking from the opposition-controlled northern town of Azaz and using an alternative acronym for the Islamic State.

The SDF is overwhelmingly dominated by Kurds, but out of sensitivity to the Arab population of Manbij, the United States has helped enlist local Arab forces in the offensive. Some of them are now questioning whether they want to remain part of the force, Sayed said.

The Turkey-based Syrian Institute for Justice, which monitors human rights violations in Syria, said it had documented 85 deaths, including entire families who were wiped out, according to Ahmed Mohammed, who works with the group and is from Manbij. He said witnesses told him the warplanes dropped eight to 10 bombs on the village early Tuesday.

“We are documenting with the families of the victims, but secretly, because ISIS will sentence anyone to death who has phone contact with us,” he said.

In a letter Tuesday to U.N. Secretary General Ban Ki-moon, the head of Syria’s opposition negotiating committee to peace talks with the government appealed for an intervention.

Describing “the unprecedented level of civilian killing in the city of Manbij,” Riyad Hijab said that “hundreds of civilians have been killed, mostly children and women. And entire families have been buried under the rubble as a result of the international coalition aircrafts’ bombing of this area.” Hijab said the casualties have been “met with suspicious international silence.”

The Syrian opposition is not allied with the SDF.

Sayed said Tokhar had a prewar population of 3,000, but the figure has been inflated by civilians fleeing the fighting in Manbij. Mohammed added that the strikes destroyed three-quarters of the village and that bodies were still being dug out from the rubble.

A Facebook page operated by activists from Manbij said the toll had reached 160, and it posted grim photographs of mangled bodies covered in dust.

The Islamic State also reported the deaths, claiming in a statement that 160 died.

Zakaria Zakaria in Istanbul and Karen DeYoung in Washington contributed to this report.