U.S.-led coalition forces launched heavy raids into Friday morning in support of Syrian fighters battling to oust ISIS from its bastion of Raqqa, activists said.
The Kurdish-Arab alliance known as the Syrian Democratic Forces broke into Raqqa city's east earlier this week, months after they launched an operation to capture the extremist stronghold.
On Thursday night, the U.S.-led coalition backing the SDF launched 25 airstrikes on Raqqa city and its outskirts, the Syrian Observatory for Human Rights activist group 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 Friday morning as the coalition tried "to undermine ISIS 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 ISIS on several fronts.
"The SDF has control of Al-Meshleb district (inside eastern Raqqa) 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 ISIS fighters as they pushed towards the city limits from the west.
Al-Meshleb was the first neighborhood in Raqqa city entered by SDF fighters.
They are expected to progress from the district into neighboring Al-Senaa and the Observatory said some of the overnight strikes targeted the area between the two districts.
ISIS 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.
"Hundreds" of U.S. military personnel are taking part in the Raqqa offensive, according to the Pentagon, which said Thursday it believed up to 2,500 ISIS fighters were still holed up in Raqqa.
Captured by the extremists in 2014, Raqqa has become synonymous with ISIS 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 ISIS rule in Raqqa, 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 caught in crossfire -
The UN children's agency UNICEF warned Friday that "an estimated 40,000 children remain trapped in dangerous conditions in Raqqa 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 Raqqa and the surrounding area since the SDF launched its Operation Wrath of the Euphrates to capture the ISIS bastion last November.
Many have described harrowing journeys as they fled Raqqa city, with ISIS fighters targeting them as they tried to escape.
Elsewhere in Syria, a U.S. warplane shot down a pro-regime drone Thursday night near the Jordanian border in the latest incident in escalating tensions in the country's south.
The U.S.-led coalition said the drone was downed after it fired at coalition forces near the Al-Tanaf garrison, where anti-ISIS 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 ISIS in the country's eastern Deir Ezzor province
