Attacks by ISIS militants have killed 51 Syrian soldiers holed up in an air base in the northern province of Aleppo this month, activists said Monday.

The Britain-based Syrian Observatory for Human Rights said government forces kept up airstrikes against ISIS in the area to hold militants at bay as the jihadi group stepped up offensives against both government forces and other insurgents it is battling in northern Syria.

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The Observatory, which tracks the conflict using sources on the ground, said at least 40 of those killed during two separate ISIS attacks on the Kweiris air base east of Aleppo city were officers.

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ISIS waged a three-day assault from August 9-12, and had begun another two days ago, the Observatory said, using car bombs in a bid to overrun the base where government forces have long been besieged by the jihadis.

The Observatory said fighting around the air base had killed at least 62 ISIS militants, including group leaders.

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The Observatory also reported that relatives of the besieged Syrian soldiers had rallied dozens of people in government strongholds in western Syria in demonstrations that urged the government to end the siege. The protests took place in areas in the coastal provinces of Latakia and Tartous, it said.