HASSAKE, Syria: The US-backed Syrian Democratic Forces militia has advanced to within four kilometers north and east of ISIS's Syrian bastion Raqqa, a spokesperson and monitor said Tuesday.

The advances bring the SDF, a Kurdish-Arab alliance, closer than ever to the extremist group's most important remaining Syrian stronghold.

But the fighters are still 40 kilometers (25 miles) from Raqqa to the west, and they do not control any territory directly to the city's south, which is bordered by the Euphrates river.

The SDF "on Monday took three villages four kilometers (two miles) north of Raqqa", spokesman Talal Sello told AFP.

The alliance also seized two more villages four kilometers to the east of the city on Monday night, he added.

"We are close to encircling Raqqa from the east and the north, but we need to advance on the western front before launching the assault (on the city) at the right moment."

Last week, the SDF seized the town of Tabqa and the adjacent dam, around 55 kilometers (35 miles) west of Raqqa, and the alliance will now push on towards their next target, the town of Al-Mansura.

The SDF has said the long-awaited attack on Raqqa would start at the beginning of the summer, probably in June.

The Britain-based Syrian Observatory for Human Rights monitor confirmed the capture of the five villages, saying the SDF had advanced by four kilometers on the northern front and two kilometers on the eastern one.

"The SDF wants to link up its troops to the north and east of Raqa," said Observatory director Rami Abdel Rahman.

ISIS fighters are still able to move in and out of the city via the area to its south, crossing the Euphrates by boat, according to SDF sources.

From there they head further south to the ISIS-held town of Al-Sukhna in Homs province, and onwards east, to Deir al-Zor province, which remains almost completely under the control of the extremists.

The SDF launched an operation to retake Raqqa city in November 2016 and it has received key backing from the US-led coalition carrying out air strikes on ISIS.