US-led coalition forces launched heavy raids into Friday morning in support of Syrian fighters battling to oust the Islamic State group from its bastion of Raqa, a monitor said.

The Kurdish-Arab alliance known as the Syrian Democratic Forces broke into Raqa city's east earlier this week, months after they launched an operation to capture the jihadist stronghold.

On Thursday night, the US-led coalition backing the SDF launched 25 air strikes on Raqa city and its outskirts, the Syrian Observatory for Human Rights monitor said.

The Britain-based group reported 23 civilians killed in the overnight strikes, including 15 who were in an internet cafe in the western suburb of Jazra, just outside the city.

Observatory director Rami Abdel Rahman said heavy strikes continued on Friday morning as the coalition tried "to undermine IS capacity and open the way for the SDF to advance further in the city's east and enter from other fronts".

SDF spokesman Talal Sello said fighters were advancing against IS on several fronts.

"The SDF has control of Al-Meshleb district (inside eastern Raqa) and is clearing it of mines and explosives at the moment," he told AFP.

He said SDF forces were also advancing on the northern front outside the city, and had repelled an attack by IS fighters as they pushed towards the city limits from the west.

Al-Meshleb was the first neighbourhood in Raqa city entered by SDF fighters. 

They are expected to progress from the district into neighbouring Al-Senaa and the Observatory said some of the overnight strikes targeted the area between the two districts.

IS fighters have been fighting back against the advancing forces with snipers as well as drones armed with explosives, according to the SDF.

They have also reportedly dug defensive trenches and tunnels to try to slow the SDF advance.

- Children caught in crossfire -

"Hundreds" of US military personnel are taking part in the Raqa offensive, according to the Pentagon, which said Thursday it believed up to 2,500 IS fighters were still holed up in Raqa.

Captured by the jihadists in 2014, Raqa has become synonymous with IS atrocities including beheadings and public displays of bodies, and also emerged as a hub for planning attacks abroad.

An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 displaced from other parts of Syria.

But thousands have fled in recent months, and the UN humanitarian office estimates about 160,000 people remain in the city.

The UN children's agency UNICEF warned Friday that "an estimated 40,000 children remain trapped in dangerous conditions in Raqa city."

"Many are caught in the crossfire," said UNICEF regional director Geert Cappelaere.

"Children are deprived of the most basic and life-saving necessities," he added, urging safe passage for those who want to leave the city.

Tens of thousands of people have fled Raqa and the surrounding area since the SDF launched its Operation Wrath of the Euphrates to capture the IS bastion last November.

Many have described harrowing journeys as they fled Raqa city, with IS fighters targeting them as they tried to escape.

Elsewhere in Syria, a US warplane shot down a pro-regime drone on Thursday night near the Jordanian border in the latest incident in escalating tensions in the country's south.

The US-led coalition said the drone was downed after it fired at coalition forces near the Al-Tanaf garrison, where anti-IS Syrian rebels are being trained.

The shooting down came after another incident earlier Thursday in which coalition forces struck "technical vehicles" advancing towards Al-Tanaf. 

It was the third time the coalition has struck pro-regime forces near Al-Tanaf in less than a month.

Syria's government is eager to deploy forces in the area and head off any dispatch of foreign-trained Syrian rebels to fight IS in the country's eastern Deir Ezzor province.

Sourc: AFP