Internally displaced boys who fled Raqqa city sell sandwiches at a camp in Ain Issa, Raqqa Governorate, Syria May 4, 2017. Picture taken May 4, 2017. (REUTERS Photo)

DAMASCUS, June 8 (Xinhua) -- The Islamic State (IS) militants are preventing civilians from fleeing the northern city of Raqqa, following a wide-scale offensive by the U.S.-led forces to liberate the city, activists said Thursday.

Around 100 vehicles transporting civilians attempted to leave Raqqa, but IS militants arrested large numbers of them and burned the cars, Raqqa-based activist group "Raqqa is Being Slaughtered Silently" said.

The group, well known for documenting the events in Raqqa, said 10 rockets hit the northern part of Raqqa.

It added that warplanes of the U.S.-led anti-terror coalition struck areas in Raqqa on Thursday, killing 14.

This comes two days after the Kurdish-led Syrian Democratic Force (SDF), with the backing of the U.S.-led coalition, announced the commencement of the fourth stage of their offensive on IS in Raqqa, which marked the first attack to storm the city itself.

The SDF said on Thursday that its forces have captured large swathes of the Mashlab neighborhood, the first neighborhood in the eastern part of Raqqa, the de facto capital of IS.

The SDF, which comprises Kurds, Arab and Assyrian fighters led by the Kurdish People Protection Units, said hundreds of Kurdish and Arab fighters have completed their training and are joining its campaign on Raqqa dubbed Euphrates Wrath to liberate Raqqa from IS.

The SDF said IS commanders are fleeing toward the Syrian Desert, where the Syrian army backed by Russia and the Lebanese Hezbollah are fighting to eliminate the IS presence in the part of the country.

The group said its forces took the villages of Rabi and Jazra and hoisted their yellow flag over the castle of Hirakel, some two km west of Raqqa.

The SDF is also fighting to capture the Division 17 military base and the Sbaiyeh neighborhood north of Raqqa.

The SDF said its forces have advanced 1.5 kilometers into Raqqa, while Kurdish activists said the SDF's push into Raqqa is quicker than expected.