ANKARA, Sept 17 (Reuters) - Turkish President Tayyip Erdogan will visit Russia later this month for talks with President Vladimir Putin about the violence in northwestern Syria, where Moscow and Ankara back opposing sides, two Turkish officials said on Friday. Turkey supports fighters who sought to topple President Bashar al-Assad, while Russia has helped shore up Assad after a decade of conflict. Both sides have complained about violations of a truce they agreed 18 months ago in the northwestern Idlib region, the last rebel bastion left in Syria, where Ankara says two Turkish troops were killed in an attack on Saturday. "The main agenda point is Syria, namely Idlib," a senior Turkish official, speaking on condition of anonymity, said of the planned talks in Russian resort of Sochi. "The conditions set out in the Idlib agreement have not been fully implemented." The March 2020 agreement followed weeks of fighting that brought Turkey and Russia close to conflict and displaced nearly a million people. "There should not be any new instability in Syria," another Turkish official told Reuters. Erdogan"s planned two-day visit will follow his trip to the United Nations General Assembly in New York next week, the officials said, without specifying exact dates. AREAS OF COOPERATION Despite backing opposing sides in both the Syrian and Libyan conflicts, Turkey and Russia have forged close cooperation in the defence, energy and tourism sectors. NATO member Turkey has bought Russian S-400 air defences, leading to U.S. sanctions on Turkish defence industries, and has been in talks with Russia over possibly buying a second batch. Both Turkish officials said this would be discussed, as well as energy projects and tourism. Ankara and Moscow were rivals in Nagorno-Karabakh during fighting between Azeri and ethnic Armenian forces last year. While Russia brokered a ceasefire between Turkey-backed Azerbaijan and Armenia, it is working with Ankara to monitor it. Turkey also angered Russia earlier this year when it sold Turkish drones to Ukraine amid tensions over the Donbass region, and later to Poland in the first such sale to a NATO member. The official said both Nagorno-Karabakh and the drone sales would be discussed in Sochi, while the senior official said that further defence cooperation may be discussed.