US-backed Syrian fighters Friday seized part of a district on the western edge of the Islamic State group's Raqa bastion and battled to advance inside the city's east, a monitor said.

The Kurdish-Arab alliance known as the Syrian Democratic Forces (SDF) broke into Raqa for the first time earlier this week, months after they launched an operation to capture the jihadist stronghold.

They are backed by the US-led coalition which carried out heavy air strikes on Raqa and its surroundings overnight and into Friday.

The Britain-based Syrian Observatory for Human Rights monitor said SDF fighters took parts of the suburb of Jazra just west of Raqa's city limits on Friday.

It said heavy clashes continued to rock other parts of the neighbourhood, where at least 15 civilians were killed Thursday night in air strikes that hit an internet cafe.

They were among 23 civilians killed in more than two dozen US-led coalition strikes on and near Raqa that night, the Observatory said.

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

"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 had also advanced on the northern front outside the city.

The SDF media department said four local journalists were wounded Friday when an IS mortar landed near them as they made their way toward Al-Meshleb.

- Fighting back -

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 areas between the two districts.

IS has 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.

"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. It has 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.

- Children in crossfire -

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.

Doctors Without Borders (MSF) also warned that Raqa's civilians were left facing a "deadly choice".

"Parents have to make an impossible decision," said MSF emergency coordinator Puk Leenders.

"Either they stay in Raqa, subjecting their children to increased violence and air strikes, or they take them over the frontline, knowing they will need to cross minefields and may be caught in the crossfire."

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.

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 what turned out to be a dud bomb at coalition forces near the At-Tanaf garrison, where anti-IS Syrian rebels are being trained.

Pentagon spokesman Navy Captain Jeff Davis said Syrian regime ally Russia helped prevent an escalation in hostilities between the coalition and the pro-regime forces.

"Russia has been very helpful and I think that the calm we see today is largely due to their efforts," Davis said.

Tensions around At-Tanaf have surged as pro-regime forces, including Iranian-backed militias, look to expand their influence.

Source: AFP