A day after announcing a formal inquiry into what watchdogs call the United States’ worst civilian casualty incident in its war against the Islamic State militant group, the US military said that more civilians may have been killed in another airstrike around the same Syrian city.

Manbij, the scene of intense fighting for over two months between Isis and US-backed militant groups, has now experienced another airstrike that “may have resulted in civilian casualties”, the US Central Command (Centcom) disclosed late on Thursday.

“We can confirm the coalition conducted airstrikes in the area in the last 24 hours,” Centcom said in a statement.

The Syrian Observatory for Human Rights, a major source of information on the plight of noncombatants caught in Syria’s devastating civil war, said it put the death toll in the latest lethal Manbij incident at 28 civilians, including, once again, women and children.

“They were killed when the warplanes of the international coalition committed a massacre in the town of al-Ghandour in the northwestern countryside of Manbij city east of Aleppo province, where the warplanes targeted areas in the town of al-Ghandour, which is more than 23 kilometers away from Manbij city, and the death toll is expected to rise because there are some people in critical situation,” the Observatory said on its English-language website on Thursday.

The Observatory said 13 others were killed in the airstrike. It is not known if those 13 are civilians or members of Isis, which has been fending off ground and air attacks for more than two months from the US and its Syrian allies.

US military officials have said they expect the battle for Manbij to drag on, as Isis has dug into a position both sides consider a critical buffer between US-backed forces and the Isis capital of Raqqa. Manbij has strategic value for another reason: it provides a pathway for Isis to exfiltrate fighters through Turkey to the outside world.

Syrian allies of the US, advancing through extreme fighting, have accumulated terabytes’ worth of digital information on Isis from computers and mobiles left behind in the battle.

On Wednesday, the US military announced that it would formally investigate an airstrike on 19 July, in the nearby village of Tokkhar, that may amount to the US’s bloodiest error of the two-year old war.

Estimates of the dead vary. But multiple Syrian sources, collected by the London-based monitoring group Airwars, put the death toll at at least 74 and at most 205. While the US military command in Baghdad suggests it might be as low as 10 dead civilians, Airwars’ chief investigator, Chris Woods, told the Guardian the credible maximum is likely to be 120-150 men, women and children dead.

The US has rejected calls by Syrian opposition figures after Tokkhar to halt the bombings for the sake of thousands of civilians trapped in Manbij, as it believes the fall of the city is critical to the overriding objective of capturing Raqqa.

Woods told the Guardian that he assesses the devastation in Manbij to stretch beyond Tokkhar. More than 520 US airstrikes since 21 May have killed between 229 and 425 civilians overall. Woods provided that estimate before Thursday’s announcement.

Centcom said it would continue a preliminary investigation of the latest Manbij incident and then “determine the next appropriate step”.

Its statement continued: “We take all measures during the targeting process to avoid or minimize civilian casualties or collateral damage and to comply with the principles of the Law of Armed Conflict.”