BEIRUT — Turkey’s president says his troops and allied Syrian fighters have reached the heart of the Islamic State stronghold of al-Bab in northern Syria and will eventually march on the extremists’ de facto capital, Raqqa.

Recep Tayyip Erdogan said Islamic State fighters have begun deserting al-Bab, which has been under attack for weeks. But the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria, says Turkish troops have yet to enter the town’s center, with the fighting still concentrated on its outskirts.

The Turkish-backed opposition forces advancing from the north are racing to seize al-Bab before Syrian government forces reach the town from the south. The Turkish-backed forces are still a long way from Raqqa, which is largely surrounded by U.S.-backed Syrian Kurdish forces.

Last week, the Kurdish-led Syria Democratic Forces launched a new phase in its Raqqa offensive, aiming to capture towns and villages east of Raqqa. The U.S.-led coalition has targeted several bridges across the Euphrates River in support of the operation.

Turkey is leading Syrian opposition forces in a broad operation called “Euphrates Shield” against both IS and the SDF. Ankara views the Kurdish group that dominates the SDF as a terrorist organization because of its links to Kurdish insurgents in Turkey.

“After al-Bab is about to be over, the period following that will be Manbij and Raqqa,” Erdogan said, referring to the northern town of Manbij, which was captured by the SDF last year after weeks of deadly fighting with the Islamic State.

“Right now al-Bab, whether by us or by the Free Syrian Army, is now besieged on all four sides and our forces along with the Free Syrian Army have entered the center,” he said.

The Observatory reported intense clashes and Turkish airstrikes and shelling.