Territory still controlled by the IS group in Raqqa is "the worst place" in Syria, the UN said Thursday, as fresh reports emerged of more civilians killed by US-led coalition air strikes.
The Syrian Observatory for Human Rights said 21 children were among at least 59 civilians killed since Monday in the air strikes aimed at dislodging the militants from Raqa.
"The worst place probably today in Syria is the part of Raqqa that is still held by the so-called IS group," the UN's humanitarian pointman for Syria, Jan Egeland, told reporters in Geneva.
The UN estimates there are up to 25,000 civilians trapped inside Raqqa, the militants group's erstwhile de facto Syrian capital.
"They are encircled by the SDF (Syrian Democratic Forces) fighters and they are used seemingly as human shields by the IS group," Egeland added, noting the "constant air raids" by the US-led coalition.
"We are therefore urging the coalition, the SDF whom we can deal with to allow as much as they can people to escape", the UN official added.
The SDF, a Kurdish-Arab alliance backed by the multinational coalition, was on Thursday battling the militants in Raqqa's Old City, of which it now controls 70 percent, according to the Britain-based Observatory.
The SDF also fought IS in the western district of Al-Dariya and the northwestern neighbourhood of Al-Barid, as well as on the outskirts of the central district of Al-Murur, it said.
The coalition has repeatedly stressed it takes every precaution to avoid civilian casualties.
But it has recognised 624 such deaths in its air strikes since 2014, a figure which many rights groups say is vastly underestimated.
The SDF launched an operation to capture Raqqa province from IS last year, and in June the alliance broke into Raqqa city for the first time.
It now holds more than half of the city, but the fighting has proved fierce and civilians have been killed both in the crossfire and while trying to flee.
Humanitarian workers believe "the situation couldn't be worse for these women, children (and) civilians who are now in this crossfire," Egeland told reporters.