This picture taken on Monday provided by The Syrian Democratic Forces (SDF) shows Syrian Democratic Forces (SDF) fighters flashing victory signs at the front line where they have been battling against Isis. (Syrian Democratic Forces, via AP)

The Syrian city of Raqqa has been liberated from Isis militants, according to U.S.-backed Syrian forces.

The city was the de facto capital of the terror regime, and its fall marks a significant blow for the jihadist group.

Syrian Democratic forces, a group comprised of Kurdish and Arab militias and backed by the U.S., drove out Isis fighters after battling in the city since June.

The U.S.-backed forces raised a victory flag inside Raqqa stadium on Tuesday morning as the months-long battle came to a close.

War monitoring group the Syrian Observatory for Human Rights said Isis was now completely cleared from the city.

Fighters of Syrian Democratic Forces gesture the “V” sign in Raqqa, Syria. (Reuters/Rodi Said)

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The fall of the city, where so-called Islamic State staged euphoric parades after its string of lightning victories in 2014, is a potent symbol of the jihadist movement’s collapsing fortunes.

From the city, the group planned attacks abroad.

A witness said militia fighters celebrated in the streets, chanting slogans from their vehicles.

A female fighter of Syrian Democratic Forces is seen in Raqqa. (Reuters/Rodi Said)

A group of militia fighters and commanders clasped their arms round each other, smiling, in a battle-scarred landscape of rubble and ruined buildings at a public square.

The flag in the stadium and others waved in the city streets bore the yellow background and red emblem of the Kurdish YPG, the strongest militia in the SDF.

Isis has lost swathes of territory in Syria and Iraq this year, including its most prized possession, Mosul, and in Syria it has been forced back into a strip of the Euphrates valley and surrounding desert.