Fighters from the Syrian Democratic Forces
Fighters from the Syrian Democratic ForcesReuters

Kurdish forces and their Arab allies in northern Syria have begun an ambitious offensive to take the de-facto capital city of the Islamic State terrorist group (ISIS).

Approximately 50,000 Kurdish and Arab fighters from the Syrian Democratic Forces (SDF) - an alliance led and dominated by the Kurdish People's Protection Units (YPG) - launched a "three-pronged offensive" on Raqqa in Syria, according to the Kurdish Rudaw news agency.

"Today our forces headed to Raqqa," a source within the SDF told the news agency, adding that US air support would be involved in the operation.

Watch: Kurdish forces announce start of Raqqa operation:

The operation will begin by tightening the noose around Raqqa, cutting off supply routes and pinning in the jihadists for a final showdown.

"Our forces will be advancing to the villages of Fatse Big, Fatse Small and Tishi, in order to clear them of ISIS militants first," he continued.

He expressed confidence that the local population would rally around anti-ISIS forces, saying Raqqa's liberation "has been demanded by the people of the city several times."

Kurdish forces are bracing themselves for a tough battle ahead, with ISIS fighters likely to fight to the death to secure the capital of their self-proclaimed "caliphate", and having had many months to prepare booby traps, mines and defensive fortifications. 

The surprise announcement of the advance on Raqqa comes just one day after Iraqi forces began their assault on Fallujah, an ISIS stronghold in that country.

Fallujah is one of two major cities in Iraq still held by ISIS, the other being its central Iraqi hub - and the country's second largest city - Mosul.

Raqqa was initially captured by Syrian rebels in March 2013, but by the next year the ISIS terrorist group had taken over, driving out or killing all rival factions.

On June, 29, 2014, the group named Raqqa the capital of its so-called "caliphate," or Islamic empire, stretching through Iraq and Syria.

A US-led coalition has been conducting airstrikes in and around Raqqa since not long after that declaration, but the Pentagon had played down talks of plans to coordinate a ground operation to seize the city from ISIS.

Two days ago those rumors resurfaced, after the United States' top military commander for Middle Eastern operations, General Joseph Votel, paid a surprise visit to northern Syria.