US-backed forces continue to advance on Raqqa, the de facto capital of the extremist "Islamic State" (IS) group, the Britain-based Syrian Observatory for Human Rights reported on Saturday.

According to the Observatory, the Syrian Democratic Forces (SDF), a coalition of Kurds and Arabs, have captured the key northeastern town of Mansoura, some 26 km (16 miles) southwest of Raqqa.

The SDF report that they are in control of 90 percent of the town, which lies halfway between Raqqa and the former IS bastion of Tabqa.

The SDF have also taken the nearby village of Heneida and the Baath Dam to the northwest of Raqqa, the Observatory reports, thus getting as close as 2 kilometers to the east of the IS stronghold and 3 kilometers to the north.

Long campaign

Local activists said the newly gained territory means that the SDF now have a clear run on Raqqa, with no more major urban communities on the way.

The Observatory gathers its information from a network of activists on the ground in the war-ravaged country.

The SDF, which receives weapons and air and ground support from the US in its campaign to defeat IS in Syria, started its offensive to capture Raqqa in November. The city has been under the control of the group since 2014.

In March, an airstrike believed to have been carried out by the US-led anti-IS coalition hit a school in Mansoura, where displaced people had taken shelter, killing at least 33.

Human rights organizations have warned of the dangers posed by launching strikes in urban areas.

Read: Civilian casualties surge as anti-'Islamic State' coalition prepares to capture Raqqa

tj/mkg (dpa, AP)