IRBIL, Iraq — Airstrikes against Islamic State-controlled villages in Syria killed dozens of civilians Tuesday in an area that is the target of a US-backed offensive, according to images and testimonies from human rights monitors and activists.

The US military said it was looking into the claims, which range from at least 56 people killed to as many as 160 dead in the village of Tokhar, about 10 miles north of the northeastern Syrian city of Manbij.

A US military statement confirmed that the US military has been conducting strikes in the Manbij area, and said investigations are under way into whether the location of the strikes that caused the casualties matches any carried out by US warplanes.


‘‘If the information supporting this allegation is determined to be credible, we will then determine the next appropriate step,’’ the statement said.

US-backed forces with the Kurdish and Arab Syrian Democratic Forces embarked on an offensive to capture the town of Manbij more than six weeks ago. On Tuesday, the US military announced that it had managed to capture the Islamic State headquarters there. The military said US warplanes had conducted 18 strikes in the previous 24 hours around the town of Manbij.

Occasionally, however, other warplanes involved in the widespread bombing of Syria stray into areas where the US military is operating and also conduct strikes, including those with the Russian and Syrian air forces.

The Syrian Institute for Justice, a Turkey-based group that monitors human rights violations in Syria, said it had documented 85 deaths, including entire families. Ahmed Mohammed, an activist with the group who is from Manbij, said witnesses in the village told him that the warplanes conducted four strikes and dropped between eight and 10 bombs early Tuesday.