Syrian rebels downed an army helicopter with a ground-to-air missile for the first time on Tuesday as it was on a strafing run around a besieged base in the northwest, a human rights watchdog said.

"It is the first time that the rebels have shot down a helicopter with a surface-to-air missile," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.

The downing came at the strategic Sheikh Suleiman base, 25 kilometres (15 miles) west of Aleppo, the last garrison in government hands between Syria's second city and the Turkish border.

Footage posted by activists on YouTube showed rebel fighters shouting: "We hit it, God is greatest," as a helicopter plunged to the ground in a ball of flames.

The Observatory said the missile was part of a consignment newly received by the rebels that had the potential to change the balance of forces in the 20-month conflict.