Smoke rises from a building in Raqqa, Syria July 31, 2017. (REUTERS photo)

DAMASCUS, Aug. 2 (Xinhua) -- At least 26 civilians have been killed over the past 24 hours in Syria's northern city of Raqqa by U.S.-led airstrikes, amid talks that the Islamic State (IS) there is in procession of chemical weapons.

Some 15 children and women were among those killed by the intensified airstrikes on IS positions in Raqqa, the de facto capital of IS, said the Syrian Observatory for Human Rights.

Tens of thousands of civilians are stuck in Raqqa, who are trying to flee that hot zone from the intensified battles spearheaded by the U.S.-backed Syrian Democratic Forces (SDF).

The airstrikes and attacks have caused great damage to the infrastructure of the city, where the IS militants are losing ground to the Kurdish-led SDF and the Syrian army, which is advancing in the countryside of Raqqa, according to the observatory.

Meanwhile, other activists placed the airstrike death toll over the past 24 hours at 62.

The Syrian Foreign Ministry sent letters of condemnation to the UN on Wednesday, saying the U.S.-led coalition is killing civilians and destroying infrastructure in Raqqa.

The ministry urged the UN to shoulder its responsibilities in putting an end to the "crimes" of the coalition.

The ministry statement came two days after the Syrian government urged the UN to work to dissolve the U.S.-led coalition in Syria.

The move comes amid ongoing military operations in Raqqa by the SDF and the Syrian army, who are fighting against the terror-designated group separately but in coordination between the United States and Russia.

State news agency SANA cited a military source as saying that the Syrian army advanced in several areas in the southern countryside of Raqqa, capturing over five villages as well as oil fields over the past 24 hours.

The report said the Syrian forces have become in control of 30 km of the southern bank of the Euphrates River in Raqqa.

The SDF forces have also made new progress, becoming in control over the southern rim of Raqqa, following the wide-scale offensive they waged two months ago to drive out IS militants.

In another report, the observatory, which said it relies on a network of activists on ground, claimed that the IS militants are in procession of chemical weapons in areas they are still controlling in Raqqa.

The London-based watchdog group added that the terror-designated group is planning to use those weapons in the last stage of the war against the SDF in the city.

The observatory voiced fears for the lives of tens of thousands of civilians as those will surely be affected should the IS chose to use such weapons.