A Russian-built plane used by the Syrian air force conducted a sarin gas attack in the spring that killed at least 83 civilians and sparked a retaliatory US strike, a UN investigation has found.

The latest report by the Commission of Inquiry on Syria also said US forces failed to take "all feasible precautions" to protect civilians in attacking alleged terrorists in Aleppo in March, destroying part of a mosque complex.

The report offers some of the strongest evidence yet of allegations that President Bashar Assad's forces conducted the April 4 attack on Khan Sheikhoun in rebel-held Idlib province.

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The US quickly launched a punitive strike on Shayrat air base, where the report says the Sukhoi-22 plane took off.

The report issued on Wednesday covers a period from March to early July.

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Syrian government officials have denied responsibility, and said last month that they would allow in UN teams to investigate.

"We have analysed all the other interpretations" of who might have conducted the attack, commission chairman Paulo Pinheiro said at a Geneva news conference. "It is our task to verify these allegations, and we concluded ... that this attack was perpetrated by the Syrian air force."

Wednesday's report, the 14th by the commission since it was set up by the UN's Human Rights Council in 2011, is based on information retrieved from satellite images, video, photos, medical records, and over 300 interviews.

The Syrian government has denied the team access to the territory it controls.

"The commission finds that there are reasonable grounds to believe that Syrian forces attacked Khan Sheikhoun with a sarin bomb at approximately 6.45am on 4 April, constituting the war crimes of using chemical weapons and indiscriminate attacks in a civilian inhabited area," the report said.

The report, which also documents violations by al Qaida's branch and other militant groups in Syria, comes at a time of considerable change in the political and diplomatic landscape and the emergence of de-escalation zones that have sharply reduced fighting in some areas.

Syrian government forces, backed by Russian and Iranian firepower and troops, on Tuesday broke a nearly three-year Islamic State siege of parts of the eastern city of Deir el-Zour. A US-led coalition is also battling the extremist group in Syria.

Syrian activists on Wednesday said fierce fighting is under way between pro-government forces and IS militants around a garrison in Deir el-Zour, after an IS counter attack late on Tuesday involving multiple suicide bombings.

The fighting was centred around the military base of the 137th Brigade on the outskirts of Deir el-Zour, where the siege had been breached the day before.

The advance of government forces in Deir el-Zour was a victory for Assad, and could soon provide relief for tens of thousands of civilians besieged by the militants since early 2015.

AP