Staffan de Mistura criticized the peace talks brokered by Russia, Iran and Turkey for not breaking the stalemate. Separately, Syria urged the West to lift sanctions in order to help migrants return.
As the latest Syrian peace conference wrapped up in Astana on Thursday, the outgoing UN representative Staffan de Mistura expressed regrets over a "missed opportunity" to speed up the creation of a key committee that would rewrite Syria's constitution.
The two-day talks in Kazakhstan's capital were brokered by Russia, Iran and Turkey.
Mistura's office said, "There was no tangible progress in overcoming the 10-month stalemate on the composition of the constitutional committee."
In turn, Russian representative Aleksandr Lavrentyev said the outcome was "positive."
"I want to say that we are sufficiently close to our cherished goal," he told reporters.
Iran, Russia, Turkey to tighten the screws
This week's peace talks were overshadowed by an apparent chemical attack on civilians inside government-controlled Aleppo. The regime accused rebels of shelling the city with chemical weapons and Russia attacked rebels' positions on Sunday in apparent retaliation, jeopardizing a 10-week-old truce.
Russia and Iran are firmly aligned with Damascus, while Turkey is backing certain rebel factions. In a joint statement on Thursday, Moscow, Tehran and Ankara said they would "step up their efforts to ensure observance" of the truce, but would also "fight against terrorism" in the Idlib demilitarized zone.
The three nations also said they would boost the work of their joint coordination center.
Syria threatens Turkey
However, Syria's UN ambassador, Bashar Ja'afari, accused Turkey of sending soldiers to a demilitarized zone, contrary to an earlier agreement to deploy police officers.
"In accordance with the agreement, the Turkish side was obliged to set up 12 police checkpoints in this area," he told reporters. Turkish forces on the ground are "no police officers, but people with heavy weapons and helicopters," Ja'afari said.
"Turkey should withdraw their army, but if they don't understand the diplomatic message, we are going to resort to other methods," Ja'afari warned, according to remarks cited in Russian by the Interfax news agency.
Migrants and Western sanctions
Commenting on the millions of Syrians who fled the country since the war started in 2012, Ja'afari said that Western nations should start lifting economic sanctions against Syria in order to facilitate their return.
"For people to go back to their jobs, to normal life, to making their country develop and flourish, the sanctions need to be lifted," he said.
The Astana talks are expected to resume in February 2019.
dj/msh (AFP, Interfax, AP)