United Nations Secretary-General Antonio Guterres described Tuesday his “enormous concern” regarding clashes between Turkish and Syrian government forces in northwestern Syria and called for the violence to stop.

Speaking with reporters in New York, the UN chief expressed his apprehension about clashes Monday that saw eight Turkish military personnel killed in an offensive by Russian-backed Syrian government forces.

“We are particularly worried that now the escalation came into a situation in which we had Turkish army and Syrian army bombing each other,” said Guterres, outlining his priorities for 2020 at an hour-long news briefing. “And that, of course, is a change in the nature of conflict that is extremely worrying, and one reason more for the cessation of hostilities, before the escalation comes to a situation that then becomes totally out of control.”

Guterres also called for armed groups to allow aid workers to reach hundreds of thousands of civilians who have been affected by fighting in Syria’s northwestern Idlib province, the last rebel-held bastion in the country’s civil war.

“We don't believe there is a military solution for the conflict in Syria. We have said it time and time again, that the solution is political, and that the process needs to move on through the Geneva talks and then through the different steps related to it,” said the UN chief.

Some 20 military personnel were killed in exchanges of fire between Turkish forces and Bashar al-Assad forces, driving up tensions between Ankara and Moscow, which back opposing sides in the war-ravaged Idlib rebel enclave.

Turkey and Russia agreed in September 2018 to turn Idlib into a de-escalation zone where acts of aggression are expressly prohibited. The Syrian regime and its allies, however, have consistently broken the terms of the cease-fire -- including a fresh cease-fire Jan. 12 -- launching frequent attacks inside the zone and killing at least 1,300 civilians since the agreement.

Turkey has complained of the carnage and continued attacks and urged Russia to rein in its ally Assad to stop the bloodshed.

Pro-Assad forces are moving north along the M5 highway that connects the capital Damascus to Aleppo, crossing Idlib.