BEIRUT: U.S.-backed Syrian forces aiming to oust Daesh (ISIS) from its Syrian stronghold Raqqa captured a ruined fortress on the edge of the city Wednesday and a U.S. coalition official said that the attack was set to accelerate.

The U.S.-backed Syrian Democratic Forces, which includes Arab and Kurdish militias, Tuesday declared the start of its offensive to seize the northern Syrian city from Daesh, which overran it in 2014.

With tens of thousands of people uprooted by the fighting, a U.N. official warned of a dire humanitarian situation, with shortages of food and fuel. The YPG group, which is part of the SDF, called for international humanitarian aid.

“We are receiving reports of airstrikes in several locations in Raqqa city,” United Nations aid official Linda Tom told Reuters by phone from Damascus.

By Wednesday, the SDF had moved into the western outskirts of Raqqa and were trying to advance into an eastern neighborhood. Shelling and airstrikes from the U.S.-led coalition hit targets around the city’s edges, according to the Syrian Observatory for Human Rights and the YPG.

West of Raqqa, the SDF cleared Hawi Hawa village and took the more than 1,000-year-old Harqalah fortress ruins, YPG militia spokesman Nouri Mahmoud told Reuters by phone.

To the east, there were clashes in the Al-Mishlab district, the first quarter the SDF entered Tuesday, Mahmoud and the Observatory said.

The Raqqa assault overlaps with the final stages of the U.S.-backed attack to recapture Daesh’s capital in Iraq, the city of Mosul.

Brett McGurk, the American envoy to the U.S.-led coalition against Daesh, said it was significant that the SDF now had a “foothold” in Raqqa.

The Islamists are “down to their last neighborhood in Mosul and they have already now lost part of Raqqa. The Raqqa campaign from here will only accelerate,” McGurk said in Baghdad Wednesday.

But he said the coalition and SDF were prepared for “a difficult and a long-term battle.”

Daesh has been forced into retreat across much of Syria. Its biggest remaining foothold is in the eastern province of Deir al-Zor, which borders Iraq.

The U.N.’s Tom said an estimated 50,000-100,000 people were trapped inside Raqqa, far fewer than its population before the Syrian war erupted in 2011. Many have fled to camps elsewhere in Syria.

In some areas around Raqqa, where the SDF has recently taken control, people had started returning home, Tom said, but yet more were still being uprooted and the situation was very fluid.

The YPG’s Mahmoud told Reuters that displaced people were coming from all edges of the city after finding their own routes out.

He said when refugees arrived at SDF positions they were being given tents and supplies, but much more humanitarian support was needed to cope with the large numbers.

McGurk said the coalition was working with the SDF on a humanitarian response.

In other developments, a military alliance fighting in support of President Bashar Assad threatened Wednesday to hit U.S. positions in Syria, warning its “self-restraint” over U.S. airstrikes would end if Washington crossed “red lines.”

The threat marks an escalation of tensions between the United States and the Syrian government and its backers over control of Syria’s southeastern frontier with Iraq, where Washington has been training Syrian rebels at a base inside Syrian territory as part of its campaign against Daesh.

The area is seen as crucial to Assad’s Iranian allies and could open an overland supply route from Tehran to Iraq, Syria and Lebanon – the “Shiite crescent” of Iranian influence that is a major concern to U.S. allies in the region.

Tensions have been growing in southeastern Syria over the last several weeks. The United States launched airstrikes Tuesday against what it said were pro-government forces who it said posed a threat to U.S. and U.S.-backed forces in the area, the second such airstrike in three weeks.

The statement from the pro-Assad alliance was issued in the name of the “commander of the operations room of the forces allied to Syria,” and was circulated by a military news unit run by the Lebanese group Hezbollah, one of Assad’s military allies.

The statement did not spell out whether Russia was a signatory.

McGurk said the United States was counting on regular contacts with Moscow to help avoid a conflict with pro-Damascus forces in the southeastern region known as the Badia.

“We really do depend upon the Russians through our deconfliction military channels ... to help work these things out, and so we hope obviously that will not happen again,” he said.

Separate remarks by Syria’s Foreign Ministry, carried by state TV, warned of the “dangers of escalation” and demanded the U.S. coalition stop strikes on Syrian soil.

The statement from the “allies of Syria” said attacks on U.S. forces could be carried out with “different missile and military systems, in the light of the deployment of American forces in the region.”

In an apparent message to show the capabilities of Damascus backer Iran, Hezbollah aired what it said was footage taken undetected by an Iranian drone of a U.S. drone flying over southeastern Syria.

The American military has repeatedly warned pro-Assad forces to stay away from a so-called deconfliction zone around their garrison near the southern Syrian town of Al-Tanf.

McGurk said the airstrike Tuesday was meant to defend American forces. The U.S.-led coalition said that ahead of Tuesday’s airstrike there had been communications with Russia over allowing pro-government forces into the deconfliction zone to evacuate personnel wounded in an attack by a militia group.

“Later, at a separate location in the deconfliction zone, the coalition observed a large Syrian pro-regime force enter the deconfliction zone,” the coalition added.

“These forces were clearly hostile and comprised a tank, artillery, anti-aircraft weapons, armed technical vehicles and more than 60 soldiers,” it said.

“The coalition issued several warnings prior to conducting the strike on two artillery pieces, an anti-aircraft weapon and a tank.”

Tensions in southern Syria have flared as the intensity of the Syrian war has died down in areas of western Syria covered by an agreement brokered by Russia, Turkey and Iran to ease the fighting.