ISTANBUL (Reuters) - President Tayyip Erdogan said on Sunday the final goal of a Turkish incursion into Syria was not just to retake the city of al-Bab from Islamic State, but to cleanse a border region including Raqqa of the jihadists.

Turkish President Tayyip Erdogan reviews a guard of honour during a welcoming ceremony at the Presidential Palace in Ankara, Turkey, February 7, 2017. Picture taken February 7, 2017. Kayhan Ozer/Presidential Palace/Handout via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. FOR EDITORIAL USE ONLY. NO RESALES. NO ARCHIVE.

Turkish-backed Syrian rebels are pressing a major offensive on al-Bab, 30 km (20 miles) south of the Turkish border. The advance risks putting them in direct conflict with Syrian government forces who are closing in on the city from the south.

“The ultimate goal is to cleanse a 5,000-square-km area,” Erdogan told a news conference before his departure on an official visit to Bahrain, Saudi Arabia and Qatar.

He said Turkish forces had no intention of staying in Syria once the area had been cleared of both Islamic State and Kurdish YPG militia fighters, whom Turkey sees as a hostile force.

The Syrian Observatory for Human Rights, a British-based war monitor, said rebels backed by heavy Turkish air strikes fought Islamic State north and southwest of al-Bab on Sunday.

Turkish forces have advanced into the city from the west in recent days, and now control around 10 percent of the city and all of its western suburbs, the Observatory said.

Syrian government forces and allied militia have also made gains south of al-Bab, near the town of Tadef, since Friday, reaching an area 1.5 km from the city, it said.

Turkey believes recent IS attacks in Turkey, including a New Year’s Day shooting in an Istanbul nightclub that killed 39 people, have been steered from al-Bab and Raqqa, and it regards a clear-out of the towns as a national security priority.

Erdogan’s spokesman Ibrahim Kalin said last week that Turkey had presented a detailed plan to oust Islamic State from Raqqa and that strategy discussions with U.S. President Donald Trump’s administration were under way.

Turkey has repeatedly said it wants to be part of the U.S.-led operation to retake Raqqa, but does not want the YPG, which has been backed by Washington, to be involved.