Officials in Turkey on February 10 told a visiting Russian delegation that attacks on Turkish observation posts in northwestern Syria must be stopped immediately and that such assaults would not remain unanswered, the Turkish presidency said in a statement.

Earlier in the day, Ankara said Russian-backed Syrian government forces killed five Turkish soldiers with an artillery attack and that Turkish forces retaliated by hitting 115 Syrian army targets including three tanks, two artillery units and a military helicopter. No reliable claims of casualties caused by that response were available. 

Turkey has forces in Syria to help resist a Syrian offensive to retake the last rebel bastion in the country after nearly nine years of civil war.

In a statement following talks between Turkish presidential adviser Ibrahim Kalin and the Russian delegation, the presidency said the Russians were told that attacks on Turkish forces were unacceptable. Moscow must fulfil its duties as mandated by a 2018 de-escalation deal between Ankara and Moscow, the Russians were further informed.

The Turkish soldiers were reportedly killed after shells struck a Turkish observation post at Taftanaz airbase in opposition-held Idlib province. Pro-Damascus government forces are mounting a major offensive there.

A week ago, eight Turkish military personnel died in a similar attack.

Syrian soldiers, supported by Iran-backed militiamen and Russian air strikes, have captured dozens of towns and villages in Idlib since December.

Almost 700,000 civilians, the vast majority of them women and children, have been displaced by the fighting, United Nations says. Turkey says it fears a massive influx of refugees. Ankara has given the Syrian army until the end of February to pull back behind 12 Turkish observation posts. These were set up in Idlib under a 2017 agreement with Russia and Iran. "We hope that the process of the regime pulling back behind our observation posts is completed in the month of February," Turkish President Recep Tayyip Erdogan has previously said. "If the regime does not pull back during this time, Turkey will have to do this job itself."

Turkey says it has sent hundreds of tanks and thousands of soldiers to reinforce the posts.

The Syrian armed forces command has called the presence of Turkish forces on Syrian soil an "illegal and a flagrant act of aggression".

James Jeffrey, the US envoy for Syria engagement and the special envoy to the global coalition to defeat Islamic State, was travelling to Ankara for talks with senior Turkish officials on the Damascus regime’s offensive in Idlib and other Syria-related issues, the US State Department said late on February 11.