RAQQA, Syria — US-backed Syrian fighters have captured almost half of the Islamic State group’s de facto capital of Raqqa, but the push into the city in northern Syria has slowed due to stiff resistance and large amounts of explosives planted by the extremists, a spokeswoman for the fighters and monitors said Thursday.
The assault on Raqqa by the Syrian Democratic Forces, a Kurdish-led fighting coalition, began June 6, backed by US-led coalition airstrikes and US 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 fall of Raqqa, the Islamic State’s self-proclaimed capital, would be a huge loss for the extremist group, which earlier this month lost the Iraqi city of Mosul. But much tougher fighting still lies ahead.
Army Colonel Ryan Dillon, the spokesman for the US-led coalition fighting the Islamic State group, said 45 percent of Raqqa is now under the control of the SDF.
In a series of tweets, he said the SDF cleared about 9 square miles of terrain this past week fighting against ‘‘stiff, sporadic resistance’’ from militants entrenched in Raqqa.
Meanwhile, senior UN humanitarian official Ursula Mueller told the UN Security Council by video from Jordan on Thursday that an estimated 20,000 to 50,000 people remain in Raqqa.
She said the city is encircled and ‘‘there is no way for them to get out.’’
Since April 1, over 200,000 people have fled their homes in the area around Raqqa, she said. The figure includes more than 30,000 displaced this month as US-backed Syrian fighters try to oust the extremists.
Nisreen Abdullah, the Kurdish spokeswoman, said 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,’’ said Abdullah, of the Women’s Protection Units or YPJ, speaking from northern Syria. ‘‘They are also using civilians as human shields and this is slowing the push as well.’’
She said the Syrian Democratic Forces, which include the Kurdish YPJ women fighters, now control 45 percent of Raqqa. She added that since the offensive began, SDF fighters have fully captured eight neighborhoods.
Plumes of smoke could be seen behind buildings in Raqqa a day earlier as the coalition pounded IS targets in the city. Syrian children looked on as US armored vehicles drove by.
Mustafa Bali, who heads the SDF media center, confirmed on Thursday that the group now has half of Raqqa and said the most important areas liberated in the past four days were the Nazlet Shehadeh and Panorama Square neighborhoods — both on the southwestern part of the city.
But he said there are IS counterattacks, militant sleeper cells, and tunnels in the area.
‘‘It was not easy, we have casualties and martyrs,’’ he said.
