In one incident, militants slit the throats of eight men accused of giving coordinates to the US-led coalition at the site of bombardment in their town.
Around 60 civilians, including 16 children and 12 women, were reportedly killed in air strikes that hit residential areas of Albo Kamal the day before.
The town sits in Deir ez-Zor Governorate – one of several major front lines against Isis as its territory in Syria continues to wane.
Days later, 36 civilians died in an Isis attack on a government-held village in Hama province.
Many, including women and children, were killed by crossfire and snipers during battles, but others were found with their throats cut, the UN said.
It raised particular concern about areas that sit on the border of Isis territory, where retreating militants and their families are mixed in with thousands of displaced people.
The High Commissioner for Human Rights, Zeid Ra’ad al-Hussein, said: “We fear civilians are in an increasingly dangerous situation as the air strikes and ground conflict intensify, possibly resulting in many more casualties, as well as retaliatory assaults by Isis against densely-populated civilian areas.”
He condemned the terrorist group for using civilians as human shields, brutally preventing them from leaving areas under its control.
“There is little doubt that Isis continues constantly to kill and endanger civilians, committing war crimes without any compunction whatsoever,” he said.
“However, it is also far from clear that the fundamental principles of international law are being properly adhered to by all the various air forces engaged in the fight against Isis.
“The same civilians who are suffering indiscriminate shelling and summary executions by Isis are also falling victim to the escalating air strikes.”
In Raqqa, where US-backed forces are advancing on Isis’s largest remaining stronghold, 23 farm workers, including 17 women, were reportedly killed in air strikes on 14 May.
There were reports of fresh deaths on Friday, as 35 people were reportedly killed when bombing destroyed a market and a four-storey building in the Isis-held town of Al Mayadeen.
Mr al-Hussein said the number of civilian deaths in bombardment was rising in Syria, particularly around frontlines in Raqqa and Deir ez-Zor, suggesting that “insufficient precautions may have been taken in the attacks”.
“Just because Isis holds an area does not mean less care can be taken,” he added.
“Civilians should always be protected, whether they are in areas controlled by Isis or by any other party.”
He urged all states operating air forces in the Syrian civil war, including the US, UK, Russia and Syria, to distinguish between legitimate military targets and civilians as required under international law.
The warning came a day after US Central Command confirmed the deaths of more than 100 civilians, including children, in air strikes on the al-Jadida district of Mosul in March.
They were sheltering in buildings targeted because of the presence of Isis snipers on the roof, on the instruction of Iraqi forces who were apparently unaware of the families inside.
Investigators said a single guided bomb hit the top floor but “ignited a large amount of explosive material” placed inside by Isis fighters, destroying the building.
Increasing rates of civilian casualties caused the Mosul offensive to be temporarily paused following the incident, which has been contested by local residents who claim there were no additional explosives inside the building.
“We call upon the international community and especially the United States to compensate the victims,” said Nuraddin Qablan, the deputy president of the Nineveh provincial council.
Civil protection rescue teams reported recovering more than 200 bodies from the area in the days following the strike, while a number of houses in the area were also destroyed by clashes between Isis and Iraqi government forces.
The bombing is the largest single instance of civilian deaths confirmed by the coalition in the nearly three-year-old campaign against Isis and brings the total number of civilians confirmed killed by the Pentagon to 457.
Independent monitoring groups put the total much higher, estimating thousands have been killed in Iraq and Syria since 2014, according to tallies kept by Iraq Body Count and Airwars.
The group has accused foreign members of the US-led coalition of seeking to conceal the deaths of civilians in their strikes by excluding them from Pentagon statistics.
Britain is among the countries claiming it has killed no civilians, despite carrying out hundreds of air strikes in Syria and Iraq as part of Operation Inherent Resolve.
Central Command’s latest casualty briefing quietly announced 80 new deaths referenced only as “attributable to coalition strikes to defeat Isis in Iraq and Syria from August 2014 to present [that] had not been previously announced” as they were carried out by non-US allies.
The US-led coalition’s statistics show that between August 2014 and this month, 4,011 airstrikes in Iraq and 404 in Syria have been performed by non-US forces.
Britain and France accounted for more than half of the attacks, while partners such as the Netherlands, Denmark, Belgium and Australia made up the bulk of the remaining missions.
Other partners including Germany provide aerial reconnaissance but do not conduct air strikes, while Jordan, the UAE, Saudi Arabia, Bahrain and Turkey have been responsible for less than 1 per cent of strikes.
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