BEIRUT/NEW YORK - A senior adviser to President Bashar al-Assad Sunday accused US-led coalition forces of carrying out an "intentional" strike against Syrian soldiers, but said Damascus remained committed to a fragile truce.

In a phone interview from the Syrian capital, Buthaina Shaaban told AFP that Assad's government "believes that the strike was intentional".

"None of the facts on the ground show that what happened was a mistake or a coincidence," she said.

On Saturday, the US-led air coalition bombed a Syrian army position near the eastern city of Deir Ezzor, killing as many as 90 soldiers.

Syrian government forces have been battling an Islamic State group offensive near Deir Ezzor since last year. "Everything was calculated and Daesh knew about it... Even Russia reached the terrifying conclusion that the United States is colluding with Daesh," Shaaban added, using the Arabic acronym for IS.

"When Daesh advanced, the raids stopped."

She said that since the US-led intervention began in Syria in 2014, "we have been saying that this is not against Daesh, that they are not striking Daesh."

Syria's army late Saturday said the raid had allowed IS to gain ground around the key Deir Ezzor airbase.

The coalition admitted that it may have hit the Syrian army, but said it believed it was targeting an IS position there. The fallout has strained a teetering ceasefire deal between the US and Russia aimed at helping to end Syria's five-year war.

Shaaban said on Sunday the truce was still in place.

"We are committed to the truce. The truce is continuing until its expiration. Maybe it will be extended, maybe there will be another agreement," she said.

Last week, the Syrian army announced that it would observe a freeze on fighting until midnight on Monday September 19. Damascus believes Saturday's raid may signal divisions within the American administration on deepening US-Russia cooperation under the truce deal, Shaaban said. "What is worrying is its (the strike's) effect on the US-Russia agreement. I believe that some elements in the United States do not want this deal," she said.

"There is a side that agrees with the Russians and another side that rejects the agreement. This makes it seem to us that the White House wants this agreement while the Pentagon rejects it."

Meanwhile, heavy clashes continued on Sunday east of Damascus in the rebel-held Jobar suburb, the Observatory and a witness said.  The al-Rahman Legion, part of a Free Syrian Army rebel alliance there, said its fighters had destroyed a government tank and killed soldiers after government forces tried to storm Jobar for the second time this week.  The Observatory said at least eight people died and many were seriously injured when helicopters dropped barrel bombs onto a town in a rebel-held part of the southern Syrian province of Daraa on Sunday.

Insurgents say they only reluctantly accepted the initial deal to relieve the dire humanitarian situation in besieged areas they control, and blamed Russia for undermining the truce. "The truce ... will not hold out," a senior rebel official in Aleppo said.

Islamic State is excluded from the truce. Separate U.S.-led, Damascus-led and Turkey-backed operations against the militants have continued throughout the ceasefire on various fronts.

On Sunday, Islamic State said it had shot down a warplane in Deir al-Zor with "anti-aircraft" guns, in the same area as the U.S.-led coalition strikes hit the Syrian military on Saturday.  The Syrian military confirmed the loss of a warplane it said was carrying out an operation against rebels.

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Syria deal must be salvaged 'at all costs': France

French Foreign Minister Jean-Marc Ayrault on Sunday blamed Syrian President Bashar al-Assad's regime for undermining the ceasefire deal brokered by Russia and the United States, and said the agreement must be salvaged "at all costs."

"We must not forget that it is first of all the (Syrian) regime, and it is always the regime, which has jeopardized the US-Russian ceasefire," said Ayrault in New York.

The faltering US-Russian deal for Syria is set to dominate the agenda as world leaders gather at the United Nations this week for the annual General Assembly meeting.

"If there is one thing that must emerge from this General Assembly meeting it is hope for peace" in Syria, Ayrault said on the sidelines of a ceremony held to commemorate the September 11 attacks.

World leaders "must latch on to this agreement and keep it alive at all costs", he said.

After months of negotiations, the United States and Russia reached the agreement on September 9 that calls for a ceasefire, the delivery of aid and the joint targeting of Islamist rebels in Syria.

A ceasefire that had been in force for nearly a week was on the brink of collapse on Sunday when rebel-held districts of Aleppo came under a barrage of air strikes.

Russia, Syria's main ally, accused the United States of endangering the deal after a US-led coalition strike that killed dozens of Syrian soldiers in the east, near the Islamic State-controlled town of Deir Ezzor.

A UN Security Council meeting held late Saturday saw the US and Russian envoys trading barbs following the airstrike, for which the United States has expressed regret.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are due on Wednesday to attend a special Security Council meeting on Syria.