Scores of people, including family members of fighters with the extremist group "Islamic State" (IS), were killed on Thursday and Friday in US-led coalition air strikes on the eastern Syrian town of Mayadeen, a group monitoring the Syrian conflict said.

Early on Friday, at least 80 relatives of IS fighters, including at least 33 children, were killed in coalition bombing raids, said the Britain-based Syrian Observatory for Human Rights, which gathered information from civilians and medical sources on the ground in the IS-held town. Coalition strikes on Thursday night killed 37 civilians, after 15 had been killed on Wednesday, the Observatory said.

Mayadeen has so far faced three days of bomb attacks.

"There were two rounds of strikes: one at Thursday night and the second after midnight, targeting buildings housing families of 'Islamic State' fighters," the Observatory's head, Rami Abdel-Rahman, told DPA news agency. He added that most of those killed were Moroccan and Syrian nationals who had fled to Mayadeen from Raqqa, the de facto capital of the group in northeastern Syria.

The bombing raids also killed some IS fighters, according to the Observatory.

Read: US plan to 'annihilate IS' raises questions over civilian toll, larger strategy

'Appalling predicament'

The United Nations' human rights chief, Zeid Ra'ad Al Hussein, called on all states flying air strikes against IS in Syria "to take much greater care to distinguish between legitimate military targets and civilians."

"The same civilans who are suffering indiscriminate shelling and summary executions by ISIL are also falling victim to the escalating air strikes, particularly in the northeastern governorates of Raqqa and Deir el-Zour," Zeid said in a statement from Geneva, using a different acronym for IS.

"Unfortunately,scant attention is being paid by the outside world to the appalling predicament of the civilians trapped in these areas," the UN officer said.

Zeid said civilians are not being spared

Rising civilian toll

Zaid's comments come as the Observatory this week reported the highest monthly civilian death toll for the US-led coalition's operations in Syria.

Coalition strikes killed a total of 225 civilians in Syria, including dozens of children, between April 23 and May 23 of this year.

The 68-member coalition is also carrying out attacks on the jihadist group in neighboring Iraq. On Thursday, a Pentagon investigation concluded that at least 105 civilians died in an anti-IS air strike on a weapons cache in the northern Iraqi city of Mosul.

Prior to this revelation, the US miltary had said coalition air strikes in the two countries had "unintentionally" killed a total of 352 civilians since 2014.

NATO announced on Thursday it would formally join the anti-'Islamic State' coalition.

tj/jm (AP, AFP, dpa)