Turkey has long sought a buffer zone in northern Syria. Critics say it is exploiting the US-led battle against Islamic State militants to strike at Kurdish forces in Iraq and Syria that it sees as a threat to Turkish rule.
The airstrikes were prompted by intelligence indicating a likely Islamic State attack. The US said a deal has been reached with Turkey to use a key airbase.
Little is known about the Khorasan group, though it is said to be a small, elite group of Al Qaeda members involved in planning high-level attacks on the US.
The humanitarian crisis is deepening and the chances of a political solution soon seem very slim.
The fighting marks the most intense rebel attack in three years in the bitterly contested city.
While the jihadis of the Islamic State hope their reputation for brutality will deter their enemies, another group's snuff video shows that what has happened instead is an expansion of viciousness and cruelty.
The group's claims that it wouldn't treat religious minorities in Syria savagely if it comes to power aren't holding up in reality.
America has been groping for a successful strategy in Iraq for more than a decade, but the realities on the ground keep getting in the way.
Iraq's largest northern city was invaded a year ago by Islamic State militants. Today, its repressive rule is intact, and the group is pushing outwards from its territory in Iraq and Syria.