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 defense 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. 

Turkish media reported Tuesday that Erdogan said the country would not allow the Syrian government to gain ground in Idlib.  

"Syria is currently trying to gain territory by driving innocent and poor people in Idlib toward our border," Erdogan was quoted as saying by the Cumhuriyet daily. "We will not give Syria the opportunity to gain territory there because this increases our burden." 

UN Secretary-General Antonio Guterres called Tuesday for an end to fighting between Turkey and Syria in Idlib. Guterres told reporters it was "extremely worrying" 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. 

(With input from AFP, Xinhua)