RAQQA, Syria (AP) — U.S.-backed Syrian fighters have captured almost half of Raqqa from the Islamic State group, but the push into the northern city has slowed due to large amounts of explosives planted by the extremists and their counteroffensives, a spokeswoman for the fighters and a monitoring group said Thursday.

The assault on Raqqa by the Syrian Democratic Forces, a Kurdish-led fighting coalition, began June 6, backed by U.S.-led coalition airstrikes and U.S. troops advising the local forces. Since then, the SDF has made steady advances from the eastern and western sides of the city reaching the walled old quarter.

The capture of Raqqa, the Islamic State group's self-proclaimed capital, would be a huge loss for the extremist group that earlier this month lost the Iraqi city of Mosul. But the liberation of Raqqa is still far from happening despite the progress.

Nazreen Abdullah, the Kurdish spokeswoman, told The Associated Press that the pace of the advance into Raqqa has slowed because of massive amounts of explosives laid by IS fighters. As the extremists become more surrounded, they have increased their suicide attacks against fighters of the SDF, she added.

"Raqqa has become a booby trapped city and this shows their (IS) weakness," Abdullah, of the Women's Protection Units, said speaking from northern Syria. "They are also using civilians as human shields and this is slowing the push as well."

She said fighters of the Syrian Democratic Forces, which includes the YPJ, now controls 45 percent of Raqqa. She added that since the offensive in Raqqa began, SDF fighters have fully captured eight neighborhoods.

The Britain-based Syrian Observatory for Human Rights said SDF fighters control half the city.

The attacks on the city have claimed many civilian casualties among the tens of thousands who are still trapped in areas controlled by IS.

The Observatory said 29 people, including eight children, were killed in airstrikes on the city on Wednesday. The activist-operated Raqqa is Being Slaughtered Silently group said 36 people were killed and more than 50 wounded in airstrikes and shelling of Raqqa.

The reports could not be independently confirmed.