ISTANBUL (Reuters) - Clashes in northern Syria have killed 15 Turkey-backed rebels fighting Islamic State in the last 24 hours, according the Turkish military, which launched a cross-border incursion in late August.

The latest fighting marks an escalation since Turkish troops crossed the border into Syria on Aug. 24 to back opposition fighters battling Islamic State. Turkey says the operation is aimed at removing the border threat the jihadists pose.

Turkish special forces and the air force are providing support to the rebels, who are engaged in intense fighting with Islamic State near the towns of Azaz and al-Rai, the military said in a statement. The operation continued on Monday, it said.

In 11 air strikes by the U.S.-backed coalition, 13 Islamic State militants were killed. The air operation is designed to push the hardline Islamists back from the Turkish border, the statement said.

The army also fired on Islamic State targets from inside Turkey after the jihadists used rockets to target the Turkish border town of Kilis, damaging buildings, it said.

Turkey says international law gave it the right to enter Syria, in an operation it has dubbed “Euphrates Shield,” to cleanse its border region of Islamic State militants who had launched attacks on Turkish cities.

It says the operation also aims to restrict Syrian Kurds allied with the United States against Islamic State from making territorial gains and moving closer to self-rule.