GENEVA (Reuters) - 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.

UN Special Envoy of the Secretary-General for Syria Staffan de Mistura, attends a meeting of Intra Syria peace talks with Syrian government delegation at the European headquarters of the United Nations in Geneva, Switzerland, March 1, 2017. REUTERS/Martial Trezzini/Pool

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.

U.S. Ambassador to the United Nations Nikki Haley called for a negotiated “path to peace” on Wednesday and said Washington stands beside de Mistura’s efforts to end the war.

“A political solution must bring the Syrian people peace and stability, free from the threat of terrorism, the despotism of their own government, and interference from self-serving foreign states,” Haley said in a statement.

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.

Britain’s special representative for Syria Gareth Bayley said the Astana talks were never meant to replace Geneva, but were a good thing if they reduced violence, improved the humanitarian situation and freed detainees.

“That means the pressure is now on the Astana process to ensure any ceasefire holds,” Bayley said in an emailed statement to mark the anniversary.

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

“The forced surrender deal on the besieged areas of Al-Waer this week, as well as continued pressure on opposition-held areas in the Damascus suburbs, are not encouraging. They highlight the need for Russia to pressure the Assad regime to uphold the ceasefire,” Bayley said.

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, and U.N. investigators say pro-Assad forces have continued to commit war crimes despite the supposed ceasefire.

De Mistura has said the Geneva talks are not hostage to progress in Astana, but they go hand-in-hand: the ceasefire needs to take hold to support the political process, and political progress is needed to make the ceasefire last.

U.N. Secretary-General Antonio Guterres called on the warring sides to “enhance (the ceasefire) further” and to make the Geneva talks succeed, while U.N. humanitarian chief Stephen O’Brien urged them 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.