* Islamist units claim victory at military base
* Syrian military strikes back with air strikes
* Videos show fighters on helicopter
By Erika Solomon and Alexander Dziadosz
BEIRUT, Jan 11 (Reuters) - Rebels seized a strategic air base in northern Syria on Friday after months of fighting, activists and insurgents said, further weakening President Bashar al-Assad's grip on the region.
Rebels had fought for the base used by military helicopters in Idlib province for months, but it only fell after Islamist units reinforced them earlier this month.
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 Qaeda.
The United States has branded Nusra a terrorist organisation 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.
In another video, men climbed atop a helicopter, hoisted assault rifles in the air and displayed a black Islamic flag. At least four corpses, said to be government pilots, lay in a dirt hole in another video.
Since the summer, rebels have been surrounding and attacking government air bases to ground warplanes and helicopters that have pounded their positions and made it harder to take and hold ground.
The Taftanaz base had been used to attack areas in the north and supply other bases in the Idlib area, the Observatory's director Rami Abdelrahman said
"Loss of the base could make it hard for the army to hold on to any control in Idlib," he said, adding all the helicopters in the base were broken and working aircraft had been moved to three other locations.
Yezid Sayigh, senior associate at the Carnegie Middle East Centre in Beirut, said Taftanaz's capture would help the rebels as they try to secure a continuous area in the north.
But he played down the broader military significance, pointing out it had taken the rebels months to take a base the government already could not use because of the fighting.
"This is a tactical gain rather than a strategic gain," Sayigh said.
Despite capturing territory across the country's north and east, the rebels have been plagued by divisions, a lack of equipment and disorganisation.
They have also seen their support from civilians erode in some areas because of poor discipline, looting and bad living conditions.
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.