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BEIRUT (Reuters) - U.S.-backed Syrian militias closed in on Islamic State’s Syrian stronghold of Raqqa on Wednesday, taking territory on the south bank of the Euphrates River with the aim of encircling the city, a militia spokesman told Reuters.

The Syrian Democratic Forces (SDF), which include Arab and Kurdish fighters and are supported with air strikes by a U.S.-led coalition, began an offensive two weeks ago to seize the northern city from Islamic State, which overran it in 2014.

Nouri Mahmoud, spokesman for the Kurdish YPG militia which is part of the SDF, told Reuters Islamic State had been ousted from the suburb of Kasrat al-Farj as the SDF moved in along the southern riverbank from the west.

When the campaign began the SDF had Raqqa, which sits on the Euphrates’ northern bank, surrounded from the north, west and east. Although Islamic State controlled the south bank of the river, coalition air strikes had destroyed the bridges connecting it to the city.

The SDF is now trying to enact a siege of the city by taking the southern bank. The forces are a couple of kilometers from achieving this aim.

The Syrian Observatory for Human Rights war monitor said on Wednesday SDF forces had moved along the southern river bank to reach the eastern edge of Kasrat al-Farj, in the area between the new and old bridges into Raqqa.

Islamic State is also facing defeat in its Iraqi stronghold of Mosul and is being forced into retreat across much of Syria, where Deir al-Zor in eastern Syria is its last major foothold.