United Nations investigators have accused the US-led coalition of breaking international law in an air strike on territory held by the Islamic State (IS) group near Raqqa in March last year.

Key points: US-backed forces broke international law in air strike near Raqqa, says UN

US-backed forces broke international law in air strike near Raqqa, says UN Investigators say the strike killed at least 150 people including civilians

Investigators say the strike killed at least 150 people including civilians They allege the forces should have known civilians were sheltering in the area

The investigators, from the Independent International Commission of Inquiry on Syria, said the strike at al-Mansoura on the night of March 20 killed "at least 150" people, as US-backed forces closed in on Raqqa, the defacto capital of the so-called caliphate.

"The international coalition failed to take all feasible precautions to protect civilians and civilian objects, in violation of international humanitarian law," they said in a statement issued overnight in Geneva.

The strike was almost immediately controversial.

While the coalition said it had killed 30 IS members, local reports alleged there were a large number of civilian victims.

Subsequent investigations by the monitoring group, Airwars, as well as Human Rights Watch, backed those claims.

The coalition maintained its own investigations did not point to civilian casualties, but the UN investigators found the coalition's targeting team should have known a large number of civilians had taken shelter at the school.

In a conclusion that casts doubt on the way the coalition investigates reports of civilian casualties, the inquiry also found "the subsequent investigation conducted by the international coalition should have been able to identify the high number of civilian casualties resulting from this incident".

"The area … was liberated almost a year ago. The coalition has had every opportunity to go and do this kind of field work itself," Chris Woods, the director of Airwars, said.

"They look like they are not responding to credible reports of civilian harm."

The coalition did not respond to requests for comment.

Russia used 'dumb' bombs on civilians

Hospitals are struggling to treat the high number of civilian casualties in Ghouta due to a lack of supplies. ( Reuters: Bassam Khabieh )

The investigators also identified a likely Russian air strike at Aitarib on November 13 as another breach of international law.

The attack killed "at least 84 individuals including six women and five children," the investigators said.

"All information available indicates that a Russian fixed-wing aircraft that took off from Humaymim airbase conducted the strikes," the statement said.

While there was no evidence the Russians deliberately targeted civilians, the investigators say they used unguided or "dumb" bombs in a densely populated area.

"This attack may amount to the war crime of launching indiscriminate attacks resulting in death and injury to civilians," they said.

"As with the [US-led] coalition that has been pressed to investigate, the Russian authorities also ought to be investigating, now that we brought this to light," Commission member Hanny Megally said.

Lack of food and medicines

In their report covering the six months to January this year, the investigators tracked the Syrian Government's tightening siege of the rebel-held region of Ghouta, east of Damascus.

Even before the intense bombardment and ground attack which is now underway, the lack of food and medicine had led to acute malnutrition with several children dying of preventable diseases, including organ failure made worse by malnutrition.

The report documents cluster bomb attacks hitting schools and kindergartens.

Some of those injured in regular bombardments died because the few hospitals and clinics could not cope.

\In May, the Government all but banned medical evacuations.

Prior to that, the rebels imposed hardline Islamic rules, requiring women and girls who needed treatment to be accompanied by a male guardian.

Hundreds of people have been killed in air strikes in Ghouta since the UN Security Council, including Russia, which is the Syrian Government's most powerful ally, demanded a ceasefire more than a week ago.

Nearly 400,000 people have been besieged there for four years.

Government officials barred medical kits from a shipment of food and other supplies for 27,500 people on Monday.

Another shipment is scheduled for tomorrow.