BEIRUT - Rebels seized a strategic air base in northern Syria on Friday after months of fighting, activists and insurgents said, further weakening President Bashar Assad's grip on the region.The Syrian military struck back hours after fighters captured the base, launching air strikes on the area, the pro-opposition Syrian Observatory for Human Rights said."Taftanaz base is being bombarded by Syrian war planes, which are trying to destroy the airport after it was seized by fighters," it said. These came from several Islamist battalions strong in northern Syria such as Ahrar al-Sham, the Islamic Vanguard and al-Nusra Front, which has links to al-Qaida.For months, rebels had fought for the base used by military helicopters in Idlib province. But it only fell after Islamist units reinforced them earlier in January.The United States has branded Nusra a terrorist organization although it enjoys wide support in Syria for its combat skills.Rebels from the Islamic Front, an alliance of several Islamist units, said Taftanaz is the largest helicopter base in northern Syria and the second largest in the country.They posted an online video showing armed men in camouflage jackets tearing down posters of Assad and shouting "Allahu akbar (God is Greatest)" at what they said was the Taftanaz base.The videos could not be verified independently. The government has imposed strict curbs on foreign media access.Another video showed men picking through crates of ammunition said to be stored in the air base's armory.The United Nations says at least 60,000 people have been killed in Syria since peaceful protests began in March, 2011 and then developed into an armed revolt after a security crackdown.