UNITED NATIONS, Feb. 1 (Xinhua) -- UN Secretary-General Antonio Guterres on Saturday called for an immediate cessation of hostilities in northwest Syria.

The UN chief is "deeply concerned" about the ongoing military operation in the region, according to a statement issued by his spokesperson.

The secretary-general underlined that attacks against civilians and civilian infrastructure, such as healthcare facilities, are unacceptable.

"Military operations of all parties, including actions against and by designated terrorist groups, must respect the rules and obligations of international humanitarian law, which include the protection of civilians and civilian objects," the statement said.

Guterres reiterated that there is no military solution to the conflict.

Hundreds of thousands of people in Idlib province have been fleeing toward Turkey as government forces attack the last major stronghold held by the opposition, the UN said, citing media reports.

In a briefing on Wednesday, UN humanitarian chief Mark Lowcock told the Security Council that the fighting in Idlib appeared "more intense than anything we have seen in the last year."