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Syrian rebels and troops clash around sprawling air base in country's north

  • This image taken from video obtained from Ugarit, which has been authenticated based on its contents and other AP reporting, shows an explosion during heavy fighting between rebels and Syrian government forces in the Barzeh district of Damascus, Syria, Friday, April 26, 2013. On the streets of Damascus, the two-year old conflict dragged on Friday, with government troops pushing into two northern neighborhoods, triggering heavy fighting with rebels as they tried to advance under air and artillery support, activists said. The Britain-based Syrian Observatory for Human Rights said the fighting between rebels and soldiers backed by pro-government militiamen was concentrated in the Jobar and Barzeh areas.(AP Photo/Ugarit via AP video)

    This image taken from video obtained from Ugarit, which has been authenticated based on its contents and other AP reporting, shows an explosion during heavy fighting between rebels and Syrian government forces in the Barzeh district of Damascus, Syria, Friday, April 26, 2013. On the streets of Damascus, the two-year old conflict dragged on Friday, with government troops pushing into two northern neighborhoods, triggering heavy fighting with rebels as they tried to advance under air and artillery support, activists said. The Britain-based Syrian Observatory for Human Rights said the fighting between rebels and soldiers backed by pro-government militiamen was concentrated in the Jobar and Barzeh areas.(AP Photo/Ugarit via AP video)  (The Associated Press)

  • This image taken from video obtained from Ugarit, which has been authenticated based on its contents and other AP reporting, shows a Syrian Army tank in the streets of the Jobar district of Damascus, Syria, Friday, April 26, 2013. On the streets of Damascus, the two-year old conflict dragged on Friday, with government troops pushing into two northern neighborhoods, triggering heavy fighting with rebels as they tried to advance under air and artillery support, activists said. The Britain-based Syrian Observatory for Human Rights said the fighting between rebels and soldiers backed by pro-government militiamen was concentrated in the Jobar and Barzeh areas.(AP Photo/Ugarit via AP video)

    This image taken from video obtained from Ugarit, which has been authenticated based on its contents and other AP reporting, shows a Syrian Army tank in the streets of the Jobar district of Damascus, Syria, Friday, April 26, 2013. On the streets of Damascus, the two-year old conflict dragged on Friday, with government troops pushing into two northern neighborhoods, triggering heavy fighting with rebels as they tried to advance under air and artillery support, activists said. The Britain-based Syrian Observatory for Human Rights said the fighting between rebels and soldiers backed by pro-government militiamen was concentrated in the Jobar and Barzeh areas.(AP Photo/Ugarit via AP video)  (The Associated Press)

Activists are reporting intense clashes in northwestern Syria as rebels attack a sprawling military air base.

The Britain-based Syrian Observatory for Human Rights says there are casualties on both sides.

Rebels have laid siege to the Abu Zuhour air base in the northwestern Idlib province for months.

The Observatory and the Local Coordination Committees said the Syrian air force conducted several air raids during Saturday's battles to ease pressure on troops.

Rebels control much of Idlib province, which borders Turkey, although government troops still hold some areas, including the provincial capital that carries the same name.

Syria's conflict started with largely peaceful anti-government protests in March 2011 but eventually turned into a civil war.

More than 70,000 people have been killed, according to the United Nations.