Turkey's president says his troops and allied Syrian opposition fighters have entered the center of the northern town of al-Bab that is a stronghold of the Islamic State group.

Recep Tayyip Erdogan said Sunday before flying to Bahrain that IS fighters have begun deserting al-Bab, which has been under attack for weeks.

The Syrian opposition's Britain-based Syrian Observatory for Human Rights, which tracks Syria's war, reported intense clashes on the northern, western and southern edges of the town amid Turkish airstrikes and shelling. It did not confirm that Turkish troops are in the town's center.

The Turkish-backed forces are racing to seize al-Bab before rival Syrian government forces arrive.

Al-Bab is 40 kilometers (25 miles) south of the Turkish border.