Four Syrian organizations say Russia bears "a high likelihood" of responsibility for 304 attacks in Aleppo that violate international humanitarian law and may constitute war crimes.

In a letter to the UN Independent International Commission of Inquiry on Syria, obtained late Thursday by The Associated Press, the groups said the attacks resulted in 1,207 civilian deaths, including 380 children.

"Evidence clearly indicates that Russia has committed or been complicit in war crimes in Syria," it said.

The letter is signed by the Syrian Civil Defence search-and-rescue group, also known as the White Helmets, the Syrian Network for Human Rights, Independent Doctors Association and Violations Documentation Center.

The organizations told the commission they would be "honoured" to provide "evidence, testimonials, and any other relevant information to assist your investigations and help the identification of suspected perpetrators."

The letter was addressed to Paulo Sergio Pinheiro, who chairs the independent commission that said in a report in February that war crimes are "rampant" in Syria and that the conflict has become "a multisided proxy war steered from abroad by an intricate network of alliances."

The four organizations said "it is vital that the commission investigate thoroughly all credible allegations of Russian violations of international humanitarian law and international human rights law."

Ask for probe of Iran's role

The 304 incidents they cited include 28 attacks against civilians including children, 101 in which children were killed, 42 using banned cluster munitions, 20 using incendiary weapons in populated areas, 51 on hospitals and medical centres, 18 on schools and 21 on mosques.

The Syrian groups urged the Geneva-based commission "to explore fully all credible accounts of Iran's complicity in war crimes in Aleppo."

They noted that Iranian-backed militias "have played a central role enforcing Syria's sieges and overseeing local forced surrender negotiations." Iranian security and intelligence services have reportedly advised and assisted the Syrian military, they said.

"It is estimated that Iran oversees some 70,000 paramilitary troops in Syria, many of whom have been involved in atrocities against civilians," the four organizations said.

"Yet nowhere has Iranian intervention been more egregious than in eastern Aleppo, where Iran deliberately obstructed a Dec. 13, 2016, cease-fire, which would have permitted thousands of innocent civilians to be safely evacuated from the besieged city," they said.

Eastern Aleppo, which had been in rebel hands since 2012, fell to the Syrian government after a relentless bombing campaign and thousands of civilians evacuated the city Thursday in a watershed moment in the five-year-old civil war hailed by Syrian President Bashar al-Assad.

Airstrikes seen as a turning point

An annex to the letter documents what the four groups said were 304 of "the most egregious" Russian violations that mainly took place between July and December in Aleppo.

Since Russian airstrikes began in October 2015, the groups said attacks on medical facilities across Syria increased from one attack every four days to one every two days. "Over the past two weeks attacks on medical facilities have increased to a rate of one every nine hours," they said.

The four organizations said they made a determination that there was a "high likelihood" of Russian responsibility for the 304 incidents based on witness testimony and corroborating evidence including video footage and identification of aircraft.

But they stressed that these incidents "represent just a fraction of the many indiscriminate acts of violence perpetrated against Syrian civilians by either the Syrian regime or Russia, where Russia could not be identified as the perpetrator with a high degree of certainty."