Any resolution to the fighting in Syria must involve President Bashar al-Assad, the United Nations envoy to Syria Staffan de Mistura said yesterday in the first such acknowledgement by the UN.

"President Assad is part of the solution," de Mistura told a joint press conference with Austrian Foreign Minister Sebastian Kurz in Vienna.

"I will continue to have very important discussions with him," he added, noting that "the only solution is a political solution."

This was the first time a UN envoy on Syria explicitly named Assad as part of a peaceful solution after nearly four years of fighting between government troops and rebels seeking the leader's overthrow.

De Mistura, who was in Damascus this week meeting with Assad, is due to deliver a report on his mission to the UN Security Council on February 17.

If no solution to the conflict is found, "the only one who takes advantage of it is (the Islamic State group) ISIS Daesh," de Mistura said, referring to the jihadists who have taken over parts of Syria and Iraq.

The group is a "monster waiting for this conflict to take place in order to be able to take advantage," he said.

Kurz meanwhile agreed that "in the fight against IS it can be necessary to fight on the same side" but insisted that "Assad will never be a friend or even a partner."

Human rights groups have accused Syria's government of indiscriminate bombardment of civilians in rebel-held areas, including with crude "barrel bombs" -- allegations Assad denied in a BBC interview this week.