A Syrian military helicopter crashed Sunday at the northern air base of Kweiras, killing all its crew, state TV said, as an activist group said it was shot down by Islamic State militants. The TV report quoted an unnamed military official as saying that the helicopter crashed Sunday as a result of a technical problem while taking off. The report did not say how many crew members were on abroad at the time of the crash. The Britain-based Syrian Observatory for Human Rights said Islamic State militants who have been laying siege to the base for months shot down the helicopter. The Observatory said one crew member was killed while the fate of the others remained unclear. Kweiras military air base is in the northern province of Aleppo and is close to the town of al-Bab, which is held by the Islamic State group. Syrian rebels have shot down helicopters in the past.