(Reuters) - A Syrian military helicopter crashed in northwestern Syria on Sunday and at least four of its crew were captured by rebels, according to a monitoring group.

Video footage on social media showed a helicopter crashing in the distance. Photographs posted showed one of the crew apparently executed on the spot and an injured pilot taken by rebels from the wreckage.

The British-based Syrian Observatory for Human Rights, which tracks violence in the country, said at least four of the six- man crew were taken hostage by the al Qaeda offshoot, the Nusra front.

Syrian television said the plane encountered a technical problem and was forced to make an emergency landing and not downed as claimed by rebels.

The United Nations says the Syrian air force uses helicopters extensively to drop barrel bombs - improvised munitions - in indiscriminate attacks that cause widespread casualties in residential areas.

Syrian President Bashar al-Assad denied in a BBC interview last February that his forces used barrel bombs.

(Reporting by Suleiman Al-Khalidi; editing by Ralph Boulton)