BEIRUT  -  An air strike on one of the last holdouts of the Islamic State group in Syria has killed 54 people, more than half of them civilians, a war monitor said on Friday.

The US-led coalition fighting the militants said it or its allies may have carried out air raids in the area, and it was investigating the alleged civilian deaths.

The raid late Thursday on an ice factory near the village of Al-Soussa close to the Iraqi border killed 28 civilians and 26 IS militants, the Britain-based Syrian Observatory for Human Rights said.

It was not immediately clear if the raid on the eastern Deir Ezzor province was carried out by an Iraqi plane or the coalition, the monitor said.

In a written statement, the US-led alliance said: "The coalition or our partner forces may have conducted strikes in the vicinity of Al-Soussa and Baghour Fukhani" on Thursday.

"We are forwarding the report to our Civilian Casualty Cell for further assessment on this allegation," it said.

Iraqi warplanes have recently carried out strikes against IS in eastern Syria, while coalition aircraft have been supporting Kurdish-led fighters battling the militants. The IS fighters were Syrians and Iraqis, the Observatory said.

State news agency SANA reported the strike late Thursday, saying more than 30 civilians were killed and accusing the coalition of carrying it out.

IS fighters swept across Syria and Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled.

They have since lost most of that territory to various offensives, but still retain pockets of land in Syria including in the country's vast Badiya desert and in Deir Ezzor. IS fighters have faced two separate offensives in Deir Ezzor on either side of the Euphrates River that cuts through the province.

Russia-backed regime forces have pushed back the militants on the western side of the Euphrates, while the US-backed Syrian Democratic Forces have battled them to its east.

Al-Soussa lies to the east of the Euphrates River, in a pocket of territory still held by the militants.

IS fighters have been expelled from most urban centres in Syria, but analysts say they have retained their ability to pounce from the desert.

Last month, an IS incursion into the town of Albu Kamal on the west of the Euphrates left dozens of pro-regime fighters dead.

Attacks spiked after the militants were evacuated from their last bastion outside Damascus in May, many heading to the Badiya desert, the Observatory said.

IS also has a presence in the northwestern province of Idlib, as well as in the southwestern province of Daraa where it has been battling opposition fighters in recent days.

The government and rebels last week announced a ceasefire agreement as the regime moves to retake control of the whole of the province, but IS is excluded from that deal.

The militants hold a corner of territory in Daraa on the border with Jordan and the Israeli-occupied Golan Heights.

Overnight Wednesday-Thursday, they seized the nearby village of Heet from rebels who had agreed to hand over their heavy weapons to the regime after deadly clashes.

Damascus raises flag in Daraa, but tough battles ahead

The rapid fall of Daraa city, the cradle of Syria's uprising, is an important victory for President Bashar al-Assad's regime, but the country's devastating war is far from over, analysts say.

Russian-backed government forces raised the flag in Daraa city on Thursday, but the regime still has two regions outside its control - and influential neighbours - to contend with.

To the west, it will have to retake the Quneitra province bordering the Israeli-occupied Golan Heights, before moving on to a major battle in the north near the border with Turkey.

"Bashar al-Assad sent a signal with the fall of Daraa city that nowhere in Syria that has risen up against him will remain outside his reach," said Nick Heras, an analyst at the Center for a New American Strategy.

It was in poverty-stricken Daraa that anti-Assad protests erupted in 2011, sparking an uprising that spiralled into a complex civil war.

Seven years into the conflict, Assad's forces have sealed a deal for a handover of the city and are determined to retake the whole of the wider province of the same name on the border with Jordan.

More than 80 percent of Daraa province has returned to regime control, the Britain-based Syrian Observatory for Human Rights monitor says, but rebels are resisting in its western countryside.

"All of these images from Daraa of Assad's flag flying are meant to hasten the process of negotiating deals" for these rebel holdouts, Heras said.

It is also intended to help the Assad regime retake the whole of southwest Syria, including Quneitra.

"The hope in Damascus is that the fall of Daraa will move the Israelis to a deal now to let Assad reconsolidate his rule in southwest Syria," he said.

But, says Sam Heller of the International Crisis Group think-tank, Quneitra will "represent a special military and political challenge".

The ICG said in a recent report that Israel had supported fighters in southern Syria since 2013 or 2014, apparently to "secure a buffer zone on its border".

This week, Israel said it had carried out missile strikes on Syrian military posts in Quneitra, after intercepting what it said was an unarmed drone that had strayed into its territory.

Syria geographer Fabrice Balanche said Damascus securing Quneitra and the adjacent demilitarised zone would be "difficult because a deal is needed with the Israelis".

"They are scared that the Syrian army will enter and then never leave," he said, adding the missiles strikes overnight to Thursday were likely a "warning".

Israel is particularly worried about the presence of Iranians next door in Syria, where they have been backing Assad's regime.

In recent months, a series of strikes in Syria that have killed Iranians have been attributed to Israel.

The regime has retaken large parts of Syria with backing from its Russian ally since 2015, but few campaigns have been as quick as the one in Daraa.

A ceasefire was announced last week between opposition fighters and the regime, less than three weeks after the start of a deadly bombing campaign.

Still "it would be a mistake for the regime to let it go to its head and think that it had definitively won the war," said Karim Bitar of the Paris-based Institute of International and Strategic Affairs.

"This war in Syria is no longer exclusively Syrian, but involves many international actors who consider they have not had their last word yet."

These include not only Israel, but also foreign actors with interests in northern Syria, where analysts say Assad's regime is likely to set its sights next.

Turkey-backed rebels hold land in the north, while US-supported Kurdish fighters are present in the northeast.

The northwestern province of Idlib, on the border with Turkey, is largely controlled by an alliance of militants and rebels led by Syria's former Al-Qaeda affiliate.

"Idlib, next up on Assad's list, promises to be far harder fighting for his forces, a mother of all battles," Heras said.

Turkey has taken in more than three million Syrian refugees displaced by the civil war, and is eager not to take in any more.

"Turkey has also indicated that for them they consider Idlib a red line," Heller said.