GENEVA/BEIRUT: Syrian President Bashar Assad’s air force conducted a sarin-gas attack that killed at least 83 civilians in April, one of 20 chemical weapons attacks perpetrated by the Syrian government in the past four years, U.N.-mandated investigators said Wednesday.

The latest report by the Commission of Inquiry on Syria offers among the strongest evidence yet of allegations that Assad’s forces carried out the April 4 attack on Khan Sheikhoun in rebel-held Idlib province in which 293 people were wounded. The United States quickly blamed the Syrian government and launched a punitive strike on Shayrat air base, where the report says the Sukhoi-22 plane that carried out the attack took off.

Syrian government officials have denied responsibility, and said last month that they would allow U.N. teams in to investigate. Russia says the U.S. and its Western allies rushed to judgment and blamed the Syrian government without visiting the site.

“We have analyzed all the other interpretations” of who might have conducted the attack, commission chairman Paulo Pinheiro said at a Geneva news conference. “And we concluded ... that this attack was perpetrated by the Syrian air force.”

Wednesday’s report is based on information retrieved from satellite images, video, photos, medical records and over 300 interviews.

The report comes as U.N.’s Syria envoy Staffan de Mistura said Syria’s opposition must accept that they have not won the six-and-a-half-year war against Assad.

De Mistura suggested the war was almost over because many countries had got involved principally to defeat Daesh in Syria, and a national cease-fire should follow soon after. The main rebel-held area, the city of Idlib, would be “frozen.”

“For the opposition, the message is very clear: if they were planning to win the war, facts are proving that is not the case. So now it’s time to win the peace,” he told reporters.

Asked if he was implying that Assad had won, he said pro-government forces had advancedmilitarily, but nobody could actually claim to have won the war.

On the ground, meanwhile, the Syrian army and its allies are fighting to secure a corridor to troops in Deir al-Zor, a day after they smashed through Daesh (ISIS) lines to break the militant siege.

The army reached Deir al-Zor city Tuesday in a days-long thrust that followed months of steady advances east across the desert, breaking a siege that had lasted three years.

Daesh counter-attacks lasted through the day, as the militants tried to repel the army, the Syrian Observatory for Human Rights said. Fierce battles raged around the city, as troops sought to expand the route and allow aid in.

“Work is progressing to secure the route and widen the flanks so as not to be cut or targeted by [Daesh],” said a commander in the military alliance backing Assad. “The next step is to liberate the city,” the non-Syrian commander said.

It points to a tough battle ahead as the army aims to move from breaching the siege to driving Daesh militants from their half of the city, the sort of street-by-street warfare in which Daesh excel.

Syrian state news agency SANA said the army had expanded its control near the corridor after heavy artillery and airstrikes.

Assad and his allies will follow the relief of Deir al-Zor with an offensive along the Euphrates valley, the commander said.

The Euphrates valley cuts a lush, populous swath of green about 260 km long and 10 km wide through the Syrian desert from Raqqa to the Iraqi border at Albukamal.

The area has been a Daesh stronghold in Syria but came under attack this year when a U.S.-backed alliance of Kurdish and Arab militias besieged and assaulted Raqqa.Rapidly losing territory in Syria and Iraq, Daesh is falling back on the Euphrates towns downstream of Deir al-Zor, including Mayadin and Albukamal, where many expect the militant group to make a last stand.

Parallel with their thrust toward Deir al-Zor, the Syrian military and its allies have been fighting Daesh in its last pocket of ground in central Syria, near the town of Salamiya on the Homs-Aleppo highway. Army advances gained control of four villages there Wednesday, further tightening the pocket, a military media unit run by Lebanon’s Hezbollah said.