At least 33 Turkish soldiers have died in a Syrian government attack in opposition-held north-western Syria, in a major escalation of the conflict. Turkey, which backs the opposition, says it hit 200 government targets in response, "neutralising" 309 soldiers. Russia, Syria's key military ally, says Turkish troops were attacked in Idlib province by Syrian forces while operating alongside jihadist fighters. The EU has warned the crisis could descend into a serious conflict. "There is a risk of sliding into a major open international military confrontation," EU foreign policy chief Josep Borrell tweeted. "It is also causing unbearable humanitarian suffering and putting civilians in danger." The Turkish and Russian presidents spoke by phone on Friday. Recep Tayyip Erdogan and Vladimir Putin both expressed concern and agreed on the need for "additional measures" to normalise the situation, with the possibility of a summit in the near future, the Kremlin says. Russian and Turkish officials are due to meet at the Turkish foreign ministry in Ankara on Friday.