The US-led coalition against so-called Islamic State (IS) has denied it was behind an attack on a Syrian town that reportedly killed dozens of civilians.

The Syrian Observatory for Human Rights said 42 people, including 11 children, died on Monday when warplanes bombed Al-Bukamal, near the border with Iraq.

IS news agency Amaq put the death toll at 25 and released video of the strike.

The coalition said it had not targeted the area on Sunday or Monday, but added that unnamed countries had done so.

“We did not conduct strikes during the time period of alleged civilian casualties,” US Army spokesman Col Ryan Dillion told Reuters news agency.

Coalition warplanes had hit only oil production facilities more than 50km (30 miles) outside Al-Bukamal on those two days, he said.

Late last month, the coalition announced that its 20,200 air strikes in Syria and Iraq since 2014 had unintentionally killed at least 396 civilians. However, human rights group believe the true figure is far higher.

Airwars, an organisation that tracks allegations of civilian deaths, estimates that between 3,290 and 5,280 civilians are likely to have died.

The Syrian Observatory, a UK-based monitoring group, said the aircraft were seen approaching from Iraq so were unlikely to have been Syrian or Russian.

They struck near a residential area and a mosque, killing 23 civilians along with 20 IS militants gathered in one building, it added.

The activist group Deir Ezzor 24 reported that the attack destroyed 15 homes and killed at least 35 civilians. It said the warplanes were believed to be Iraqi.

Also on Tuesday, the UK Ministry of Defence said an RAF Reaper drone had stopped IS militants from carrying out summary killings in Al-Bukamal on 9 May.

The drone’s pilot observed a large crowd of civilians in one of the main streets and then a van unloading two shackled prisoners in front of them, a statement said.

The militants beside the crowd could not be targeted, so the drone fired a Hellfire missile at two stationed as sentries on a nearby roof, it added. Afterwards, the militants fled the scene and the crowd dispersed.

In a separate development, the Organisation for the Prohibition of Chemical Weapons (OPCW) said the blister agent sulphur mustard had been used in an apparent attack in Aleppo province on 16 September last year.

The watchdog told the UN Security Council that as a result of interviews and analysis of blood samples taken by the Syrian government and Russia, it could confirm that two female casualties reportedly involved in the incident in the Kurdish village of Um Hosh had been exposed.

A mortar shell recovered by Russian investigators also contained sulphur mustard.

The OPCW’s fact-finding mission is not tasked by the Security Council with determining who was behind the attack, but Kurdish officials blamed IS militants.

The mission has previously concluded that IS used sulphur mustard in an attack in August 2015, and that Syrian government forces used chlorine as a weapon at least three times between 2014 and 2015.

It also says the nerve agent Sarin was used in the rebel-held town of Khan Sheikhoun on 4 April. More than 80 people were killed in a suspected Syrian government air strike – an incident President Bashar al-Assad says was fabricated.