Russia is not discussing the future of Syrian President Bashar Assad in its talks with Iran and Turkey, Deputy Foreign Minister Sergei Ryabkov said on Thursday.

The foreign and defense ministers of Russia, Iran and Turkey met in Moscow on Tuesday and agreed to help broker a new peace deal for Syria.

On Wednesday, clashes between Turkish-backed Syrian rebels and Islamic State militants intensified around the northern Syrian town of al-Bab on Wednesday, killing 14 Turkish soldiers and 138 jihadists, the army said.

The fighting, in which 33 Turkish soldiers were also wounded, is some of the deadliest yet in Turkey's nearly four-month-old "Euphrates Shield" operation in northern Syria.

"The operation to control al-Bab, which is being besieged under the Euphrates Shield Operation, is on-going," the army said in a statement.

The military had said earlier that the rebel forces, which have been launching attacks on Islamic State fighters in al-Bab for weeks, had largely established control over the strategic area around the town's hospital.

"Once this area has been seized, Daesh's dominance of al-Bab will to a large extent be broken," it said in an earlier statement on Wednesday, using an Arabic acronym for the group. Islamic State was using suicide bombers and vehicle-borne explosives intensively, it added.