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Syrian troops besiege IS held northern stronghold

  • This frame grab from video provided by the government-controlled Syrian Central Military Media, shows Syrian troops taking up positions during fighting with insurgents, in an eastern neighborhood of Damascus, Syria, Wednesday, March. 22, 2017. Syrian troop are engaged in fighting with insurgents who have tried to breach government lines in eastern neighborhoods of the capital this week. The operation is spearheaded by the al-Qaida-linked Levant Liberation Committee. (Syrian Central Military Media, via AP)

    This frame grab from video provided by the government-controlled Syrian Central Military Media, shows Syrian troops taking up positions during fighting with insurgents, in an eastern neighborhood of Damascus, Syria, Wednesday, March. 22, 2017. Syrian troop are engaged in fighting with insurgents who have tried to breach government lines in eastern neighborhoods of the capital this week. The operation is spearheaded by the al-Qaida-linked Levant Liberation Committee. (Syrian Central Military Media, via AP)  (The Associated Press)

  • This frame grab from video provided on Wednesday March 22, 2017, by Ibaa news agency, the communications arm of the al Qaeda-linked, Levant liberation committee, outlet that is consistent with independent AP reporting, shows black smoke rising after a suicide bomber from the militant group of Levant Liberation Committee, blowup himself at a Syrian government forces position, in Souran Town, northern Hama, Syria. Insurgents advanced on government-held towns and positions north of the central city of Hama. An al-Qaida-linked group spearheaded the assault, launched Tuesday, by detonating a car bomb in the nearby town of Souran. (Ibaa News Agency, via AP)

    This frame grab from video provided on Wednesday March 22, 2017, by Ibaa news agency, the communications arm of the al Qaeda-linked, Levant liberation committee, outlet that is consistent with independent AP reporting, shows black smoke rising after a suicide bomber from the militant group of Levant Liberation Committee, blowup himself at a Syrian government forces position, in Souran Town, northern Hama, Syria. Insurgents advanced on government-held towns and positions north of the central city of Hama. An al-Qaida-linked group spearheaded the assault, launched Tuesday, by detonating a car bomb in the nearby town of Souran. (Ibaa News Agency, via AP)  (The Associated Press)

  • This frame grab from video provided by the government-controlled Syrian Central Military Media, shows a tank of Syrian troops passing by the remains of a burned bus, in an eastern neighborhood of Damascus, Syria, Wednesday, March. 22, 2017. Syrian troop are engaged in fighting with insurgents who have tried to breach government lines in eastern neighborhoods of the capital this week. The operation is spearheaded by the al-Qaida-linked Levant Liberation Committee. (Syrian Central Military Media, via AP)

    This frame grab from video provided by the government-controlled Syrian Central Military Media, shows a tank of Syrian troops passing by the remains of a burned bus, in an eastern neighborhood of Damascus, Syria, Wednesday, March. 22, 2017. Syrian troop are engaged in fighting with insurgents who have tried to breach government lines in eastern neighborhoods of the capital this week. The operation is spearheaded by the al-Qaida-linked Levant Liberation Committee. (Syrian Central Military Media, via AP)  (The Associated Press)

Syrian government forces are besieging the last Islamic State stronghold in the northern province of Aleppo, weeks after launching an offensive to free the entire province from militants.

The push on Deir Hafer comes as U.S. aircraft ferried Syrian Kurdish fighters and allies behind IS lines to spearhead a major ground assault on the strategic IS-held town of Tabqa in Raqqa province, which borders Aleppo.

That airlift marked a deepening U.S. involvement in Syria's conflict ahead of a looming battle for IS's de facto capital, the city of Raqqa.

The Syrian Central Military Media says Syrian troops launched the Deir Hafer siege late Wednesday after capturing nearby areas.

The opposition's Britain-based Syrian Observatory for Human Rights says Syrian troops have now cut the road linking Deir Hafer with Raqqa province.