GENEVA: Negotiations to end the war in Syria must speed up, U.N. Syria envoy Staffan de Mistura told Reuters on Wednesday, on the sixth anniversary of the start of the conflict.

So far talks to end the war, which has killed hundreds of thousands, driven 5 million into neighboring countries and left 13.5 million in urgent need of humanitarian aid, have gone almost nowhere.

"It’s becoming one of the longest and most cruel wars of recent years," de Mistura said. "That’s why there is a need for an acceleration of any type of negotiations – in Astana, in Geneva, in New York, wherever."

De Mistura is trying to mediate a political agreement between Syria's warring sides, and after a procedural round of talks in Geneva ended on March 3, he plans to bring the negotiators back for in-depth discussions on March 23.

Complementary talks on Syria's shaky ceasefire and confidence-building measures such as prisoner releases are now nearing the end of a third round in Kazakhstan's capital, Astana. They have made no real progress, either.

The Astana talks are run by Russia, Turkey and Iran, backers of the warring sides, and have little U.N. involvement.

Rebel groups say they cannot be expected to negotiate if the ceasefire is not observed. They blame forces loyal to Syrian President Bashar Assad for continually breaking it.

Assad has made major advances on the battlefield with Russian and Iranian help, but the U.N. has repeatedly warned there can be no military solution to the war.

De Mistura has denied that the U.N. talks in Geneva are hostage to progress in Astana. He considers them complementary: the ceasefire needs to take hold to support the political process, and the political process needs to move forward to make the ceasefire last.

U.N. humanitarian chief Stephen O'Brien said the warring sides and their backers needed to "regain their sense of humanity."

"We join Syrians in hoping that 2017 will be the year the carnage finally ends," O'Brien said in a statement to mark the anniversary.