By Humeyra Pamuk and Orhan Coskun

ISTANBUL/ANKARA (Reuters) - Turkish President Tayyip Erdogan discussed an attack on Turkish troops in Syria with his Russian counterpart Vladimir Putin on Friday as Turkish-backed rebels pressed an offensive to take the Syrian city of al-Bab from Islamic State.

The Turkish military has said Thursday's air strike, which killed three of its soldiers, was thought to have been carried out by the Syrian air force. It would be the first time Turkish soldiers have died at the hands of Syrian government forces.

Russia is Syrian President Bashar al-Assad's main military backer, while Turkey backs the rebels fighting to oust him.

Erdogan told Putin that Turkey respected Syria's territorial integrity and that its military incursion, launched in August to repel Islamic State from the border, showed its determination to fight militant groups, sources in the Turkish presidency said.

The Kremlin said the discussion on Syria was constructive and that both sides agreed to continue active dialogue to coordinate efforts against international terrorism.

The Turkish sources said both leaders also agreed to try to resolve the humanitarian crisis in Aleppo, where a government siege of the rebel-held east, aggravated by renewed, frequent air strikes on hospitals in the past week, have left residents desperately short of medicines, food and fuel.

Rebels in east Aleppo have agreed to a U.N. plan for aid delivery and medical evacuations, but the United Nations is awaiting a green light from Russia and the Syrian government, the U.N. said on Thursday.

RISK OF ESCALATION

The killing of the Turkish soldiers on Thursday - the first anniversary of Turkey's downing of a Russian jet over Syria - raised fears of an escalation in an already complex battlefield.

Ankara and Moscow only restored ties, which had been damaged by the jet incident, in August. They continue to pursue conflicting goals in Syria, although Turkey has of late been less openly critical of Assad than in the past.

The advance by largely Turkmen and Arab rebels backed by Turkey toward al-Bab, the last urban stronghold of Islamic State in the northern Aleppo countryside, potentially pits them against both Kurdish fighters and Syrian government forces.

Another Turkish soldier was killed and five wounded in clashes with Islamic State on Friday, the military said.

The latest casualties bring the number of Turkish soldiers killed in Syria to 17 since Ankara launched an incursion three months ago to try to push Islamic State and Kurdish fighters from Syrian territory along its border.

The Turkish military also said four Syrian rebels had been killed and 25 wounded in clashes in the 24 hours to Friday morning. Turkish fighter jets were continuing to strike Islamic State targets near al-Bab, it said.

Al-Bab is of particular strategic importance to Turkey because Kurdish-dominated militias have also been pursuing a campaign to seize it. Ankara is determined to prevent Kurdish forces from joining up cantons they control along the Turkish border, for fear it will stoke Kurdish separatism at home.

Turkey is backing the Syrian rebels with troops, tanks and artillery, as well as reconnaissance flights along the border. Washington has said the U.S.-led coalition, of which NATO member Turkey is a part, is not providing support for the operation.

(Additional reporting by Tulay Karadeniz in Ankara and Vladimir Soldatkin in Moscow; Writing by Nick Tattersall; editing by Mark Heinrich)