Sousa (Syria) (AFP) - The Islamic State group's once-sprawling "caliphate" has been reduced to a four-square-kilometre pocket of territory in eastern Syria, a senior Kurdish commander said Monday.

With support from a US-led military coalition, the Syrian Democratic Forces (SDF) are in the final stages of an assault launched more than four months ago against the jihadists' last bastion.

A dwindling number of IS fighters, led mostly by Iraqi commanders, are now defending only a handful of hamlets in the Euphrates Valley, SDF commander Heval Roni said.

"Geographically speaking, there are only four square kilometres left under IS control, stretching from Baghouz to the Iraqi border," he told AFP in the Baghouz area.

"There are some high-ranking IS leaders among them... but we don't know who exactly," said Heval Roni, who heads SDF operations in the area.

The SDF is a Kurdish-led force that also includes Arab fighters from the region and which has spearheaded the fight against IS in Syria since it was formed in 2015.

The commander said he had no information about IS supremo Abu Bakr al-Baghdadi, who is believed to still be alive and is the world's most wanted man.

In an interview to AFP last week, the top commander of the SDF said that the battle was winding up but that his forces would need about a month to assert full control over the area and declare victory.

According to the Syrian Observatory for Human Rights, more than 1,200 jihadists and around half as many SDF fighters have been killed since the start of the offensive on September 10.

The Britain-based monitoring group says more than 400 civilians have also perished, many of them killed by coalition air strikes.