ISTANBUL  :  Turkey will respond “in the firmest possible manner” to any new attack against its forces by the Syrian government, President Recep Tayyip Erdogan told Russian leader Vladimir Putin, Turkish officials said Tuesday.

Erdogan said in a phone call that Ankara would “continue to use its right of legitimate defence in the firmest possible manner”, the presidency said a day after Turkish and Syrian forces engaged in their deadliest clashes since Ankara sent troops to Syria in 2016.

Syrian government shelling of Turkish positions in the opposition-held province of Idlib killed at least five Turkish soldiers and three civilians, Ankara said.

Retaliatory fire from Turkey killed at least 13 Syrian government troops, according to the Britain-based Syrian Observatory for Human Rights.

The fighting “dealt a blow to joint (Russian-Turkish) efforts aimed at bringing peace to Syria,” Erdogan told Putin, according to the Turkish presidency.

Moscow, which backs the government of Bashar al-Assad, and Ankara, which supports some of the rebel groups opposed to it, are the two main foreign brokers in the nearly decade-old Syrian conflict.

UN chief calls for end to fighting

UN Secretary-General Antonio Guterres called Tuesday for an end to fighting between Turkey and Syria in Syria s opposition-held province of Idlib on Tuesday.

Guterres told reporters it was an “extremely worrying” escalation in the conflict that the Turkish army and the Syrian army were now “bombing each other” in the restive northwest region.

“My strong appeal is for a cessation of hostilities,” he said.

“We don t believe there is a military solution for the conflict in Syria. We have said time and time again that the solution is political,” Guterres added.

On Monday, the Turkish and Syrian armies engaged in their deadliest clashes since Ankara sent troops to Syria in 2016.

Regime shelling of Turkish positions in Idlib killed at least five Turkish soldiers and three civilians, Ankara said.

Retaliatory fire from Turkey killed at least 13 Syrian government troops, the Britain-based Syrian Observatory for Human Rights said.

The incident further tested the uneasy coordination between Russia and Turkey — the two main foreign brokers in the Syrian conflict.

Russian-backed Syrian government forces and militia have retaken dozens of villages in the last rebel enclave in recent weeks, pushing displaced populations ever closer to the Turkish border.

On Tuesday, Turkey s President Recep Tayyip Erdogan said Ankara would not allow Syrian forces to gain further ground.