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 escalate further.
"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."