Rebels in Syria say they have launched a major offensive to capture a key airbase in the north of the country.

Video posted on the internet showed fighters attacking the Taftanaz base in an attempt to control a strategic zone between Syria's two biggest cities, Aleppo and Damascus.

The attack comes a day before a crucial opposition meeting in Doha, Qatar.

It also follows the apparent murder of a number of Syrian troops by rebels, which drew international condemnation.