Rebels in Syria claim to have launched a major offensive against a key government airbase.
The facility in Taftanaz is of immense strategic importance as it covers the city of Aleppo and the capital Damascus.
The raid began at dawn with several units of the Free Syrian Army and the radical Islamist al-Nursa Front.
Rebels bombarded the complex with rockets and mortars.
Reports suggest that Syrian government forces responded with air attacks, those opposed to the Assad regime say many civilians have been killed as a result.
The assault comes a day before a crucial opposition meeting in Doha, Qatar.
The US is keen for the disparate groups to find some kind of unity and bring about a successful conclusion to the 20-month uprising that has claimed the lives of some 36,000 people to date.