The U.N. envoy on Syria has launched his latest effort to bring that country's parties together toward a long-elusive agreement to end the conflict there.

Staffan de Mistura on Tuesday announced the heads of four "working groups" on key issues ahead of his discussions in the coming days on the sidelines of the U.N.'s annual gathering of world leaders.

De Mistura is expected to meet with the five permanent members of the U.N. Security Council and the U.N. secretary-general on a way forward amid a Syrian refugee crisis and new Russian military moves in Syria. More than a quarter of a million people have been killed in the conflict, which is now well into its fifth year, and an estimated 4 million people have fled the country.

The groups will hold simultaneous discussions among Syrian parties on safety and protection of civilians; political and constitutional issues; combatting terrorism and military and security issues; and reconstruction and development.

A European leads each working group. Norwegian Refugee Council secretary general Jan Egeland, who served as U.N. humanitarian chief from 2003 to 2006, leads the group on safety and protection of civilians.

"We're failing the Syrians — that's the honest truth here," Egeland told The Associated Press earlier this year.

"This is the defining humanitarian challenge of our times," said de Mistura, whose previous approaches as the U.N. envoy have failed to move forward amid bitter divisions among both Syrians and the international community over, among other issues, whether Syria's President Bashar Assad must go.