Turkey's President Recep Tayyip Erdogan has said the final goal of a Turkish incursion into northern Syria is to clear a 5,000sq-km "safe zone", vowing to press on towards ISIL's self-declared capital in the country, Raqqa.

Turkish troops and their Syrian rebel allies had entered the centre of the ISIL bastion Al Bab, Erdogan said on Sunday, adding that its capture was just a "matter of time".

"After Al Bab is about to be over, the period following that will be Manbij and Raqqa," Erdogan told journalists before his departure on an official visit to Bahrain, Saudi Arabia and Qatar.

"We shared our thoughts with the new US administration and CIA and we will follow the developments in line with our stance," he added.

"The ultimate goal is to establish a safe zone by cleansing a 4,000 to 5,000sq km area from the terrorists."

ISIL, which stands for Islamic State of Iraq and the Levant, and is also known as ISIS, captured Raqqa in northern Syria in March 2013.

Erdogan said ISIL fighters had begun deserting Al Bab, but the Syrian Observatory for Human Rights said Turkish troops have yet to enter the town's centre.

The UK-based war monitor, which relies on a network of activists inside Syria, said Turkish forces had advanced into Al Bab from the west in recent days, and now controlled around 10 percent of the town and all of its western suburbs.

READ MORE: Syria operations 'will continue until the end', says Erdogan

The Turkish army launched a campaign inside Syria on August 24, codenamed Euphrates Shield, to clean its border from ISIL fighters and stop the advance of Kurdish militia, seen by Turkey as a "terrorist" hostile force.

Erdogan said Turkish forces had no intention of staying in Syria once the area had been cleared of both ISIL and Kurdish YPG fighters.

The Turkish-backed opposition forces advancing from the north are racing to seize Al Bab, ISIL's last stronghold in the northern province of Aleppo. 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 US-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 the city. The US-led coalition has targeted several bridges across the Euphrates River in support of the operation.

"The potential for further conflict is clear to see, unless some sort of solution has been agreed between Turkey, the US, Russia and the Kurds," Al Jazeera's Andrew Simmons, reporting from Gaziantep, on the Turkish side of the Syria-Turkey border, said.

Simmons said a border buffer zone, safe areas and no-fly zones had all been proposed by Turkey in the past, but rejected by the administration of ex-US President Barack Obama.

"Now, have things changed? Has US President Donald Trump agreed to something? It isn't clear at this stage, but Erdogan is insisting that he suggested cities being built in this safe area and refugees coming back from Turkey to be settled in them," Simmons said.

READ MORE: Displaced Syrians stuck in limbo at Idlib's Taiba camp

The clashes in Al Bab have proved the toughest so far for Turkey's army, with soldier fatalities increasing.

One Turkish soldier was killed and three others wounded in clashes with fighters on Sunday, the private Dogan news agency reported, citing military sources.

The latest casualty raised the number of Turkish troops killed in the Syria offensive to 67.

The three injured troops were evacuated from Al Bab and taken to hospitals in Turkey's Kilis and Gaziantep provinces near the Syrian border, Dogan reported.