Dozens of civilians, including children, were killed on Monday and Tuesday by U.S.-led airstrikes in Syria.

The strikes appeared to have been a mistake, with the civilians taken for Islamic State (IS or ISIS) militants, the U.K.-based human rights group the Syrian Observatory for Human Rights group told the AFP news agency.

Fifty-six civilians were killed on Tuesday by coalition forces, and 21 civilians were killed by the coalition on Monday. The 77 civilian deaths included at least 11 children.

The BBC reported Tuesday:

At least 56 civilians have died in US-led coalition air strikes near the Islamic State stronghold of Manbij in north Syria, opposition monitors say. The Syrian Observatory for Human Rights said residents had been fleeing the village of Tokhar when they were hit. An opposition activist network said 90 had died in Tokhar and nearby Hoshriya. There was no immediate comment from the coalition, which has been providing air support for the Kurdish-led offensive to drive IS militants out of Manbij.

On Monday, France24 reported:

Air strikes by the U.S.-led coalition killed at least 21 civilians in and around a stronghold of the Islamic State group in northern Syria on Monday, a monitor said. At least 15 civilians were killed in raids in a northern district of Manbij while six others were killed in a village near the city, according to the Syrian Observatory for Human Rights.

The UN Commission on Human Rights warned Friday that 70,000 civilians were likely trapped in Manbij, where the international body believes the situation is "deteriorating dramatically as fighting continues between ISIL and the Syrian Democratic Forces (SDF) which is being supported by airstrikes."

Before the airstrike in Tokhar on Tuesday, Airwars, a website tracking U.S.-led coalition killings of civilians in Syria, said this is the "worst ever week" for deaths caused by the coalition in the two years since the conflict started.

The U.S. has in recent months been intensifying its airstrikes in Syria, as journalist and radio host Sonali Kolhatkar wrote in May.

The Syrian Network for Human Rights, an advocacy group based in the U.S. and the U.K, reported on social media that by Tuesday evening local time the civilian death toll from the day's strikes had reportedly climbed to 65, although the rights group has yet to verify that number:

At least 65 civilians died in #IntlCoalition shelling on Al Toukhar in #Aleppo, SNHR yet to verify it, Jul 19 pic.twitter.com/heDf0PNvce — Syrian Network (@snhr) July 19, 2016

Airwars reported that nine families were among those killed by the U.S.-led coalition on Tuesday:

Nine families killed in early morning alleged Coalition strikes at Tokhaz July 19, with victims buried in mass grave https://t.co/5fPFGmQF15 — Airwars (@airwars_) July 19, 2016

Middle East Eye reports that the coalition did not respond to requests for comment on the recent civilian deaths. The news organization added:

Raed Saleh, the leader of the Syrian Civil Defence Force which conducts humanitarian rescue missions in the rebel-held areas, told Middle East Eye in June that he had confronted the U.S.-led coalition about civilian deaths in September 2014. "Mistakes are likely to happen," Saleh said he was told at the time.

Airwars estimates that the total number of civilians killed in Syria by the U.S.-led coalition is 1,422, at minimum, to date.