Updated

U.S.-backed Syrian Democratic Forces sealed off the last open road out of Raqqa on Thursday, encircling the city that served as a hub for the Islamic State, according to monitors.

The Syrian Observatory for Human Rights reported that SDF fighters captured villages on the southern bank of the Euphrates River, including Kasrat Afnan and Kasab.

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The SDF has been trying to break into ISIS territory surrounding Raqqa for months. The first major break was on June 6. In the time since they have captured four districts, two east and two west. They have been bearing down on the north but hadn't had much luck with the southern border.

SDF/US in Raqqa

U.S. forces patrolling on the outskirts of the Syrian town of Manbij, in Aleppo province, in March 2017. (Arab 24 network, via AP, File)

The southern approach has been the slowest thus far. “The SDT has been able to completely encircle Raqqa,” Syrian Observatory head Abdel Rahman said after Thursday’s advance.

Abdel Rahman told AFP the forces, “cut the last route IS used to withdraw from Raqqa towards territory it controls in the Syrian desert and in Deir Ezzor province."

Col. Joe Scrocca, spokesman for the U.S.-led coalition, told The Associated Press that encircling the city "has been the SDF plan from the start."

ISIS has held Raqqa since 2014 and declared it the de facto Syrian capital of its self-declared “caliphate” three years ago on June 29.

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Some of the worst atrocities, from public beheadings to planning overseas attacks have taken place in Raqqa. The United Nations has reported that up to 100,000 civilians remain trapped in the city.

The Associated Press contributed to this report.