ANTAKYA, Turkey — Once one of Syria’s closest allies, Turkey is hosting an armed opposition group waging an insurgency against the government of President Bashar al-Assad, providing shelter to the commander and dozens of members of the group, the Free Syrian Army, and allowing them to orchestrate attacks across the border from inside a camp guarded by the Turkish military.

The support for the insurgents comes amid a broader Turkish campaign to undermine Mr. Assad’s government. Turkey is expected to impose sanctions soon on Syria, and it has deepened its support for an umbrella political opposition group known as the Syrian National Council, which announced its formation in Istanbul. But its harboring of leaders in the Free Syrian Army, a militia composed of defectors from the Syrian armed forces, may be its most striking challenge so far to Damascus.

On Wednesday, the group, living in a heavily guarded refugee camp in Turkey, claimed responsibility for killing nine Syrian soldiers, including one uniformed officer, in an attack in restive central Syria.