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 regime 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 regime 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 regime forces and militia have retaken dozens of villages in the last opposition 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.