Meanwhile, Turkish forces detained 18 people, suspected of being part of Syria's regime forces, in the northern town of Ras al Ayn, Ankara said.
The foreign ministers of Turkey, Russia and Iran reaffirmed their strong commitment to the sovereignty, independence, unity and territorial integrity of Syria in a joint statement on Tuesday.
They issued the statement as guarantor states of the Astana process on the peaceful settlement of the Syrian crisis ahead of the first meeting of the Constitutional Committee of Syria, which consists of Assad regime, opposition and civil society members.
UN Special Envoy for Syria Geir Pedersen had earlier met with Turkish Foreign Minister Mevlut Cavusoglu, his Russian counterpart, Sergey Lavrov, and Iranian Foreign Minister Mohammad Javad Zarif ahead of the committee's inaugural meeting Wednesday.
They expressed their conviction that the launch of the Constitutional Committee proved "there could be no military solution to the Syrian conflict."
They said they were committed to advancing a viable and lasting Syrian-led, Syrian-owned and UN-facilitated political process in line with UN Security Council resolution 2254, which was backed by all members of the Security Council.
Cavusoglu said Turkey believes its Russian partners regarding the withdrawal of the YPG from the safe zone area in northern Syria but felt otherwise about the terrorists.
YPG is the Syrian branch of the PKK which is recognised as a terrorist organisation by Turkey, US and the UK. The group occupies near one-third of Syrian territory.
"Russia informed us that PKK/YPG elements left. We have to believe our Russian partners, but we cannot trust terrorists," Cavusoglu said at the press conference in Geneva with his Russian and Iranian counterparts.
He said the launch of talks for a new Syrian constitution is a huge step forward.
On October 9, Turkey launched Operation Peace Spring to eliminate terrorists from northern Syria east of the Euphrates River to secure Turkey's borders, aid in the safe return of Syrian refugees and ensure Syria's territorial integrity.
Ankara and Moscow reached a deal on October 22 under which YPG/PKK terrorists will pull back 30 kilometres (near 20 miles) south of Turkey's border with northern Syria within 150 hours and security forces from Turkey and Russia will conduct joint patrols there.
"This is a huge joint victory," said Lavrov, noting that there were "a lot of difficulties" to get to the start of the constitutional committee, which confirms the "commitment to the territorial integrity of Syria."
"There is no military solution for the Syrian conflict," said Lavrov.
Zarif agreed that the constitutional committee is the beginning of "a very difficult process."
"The commission must be Syrian-owned and led and must be acceptable to the Syrian people," he said, and not "foreign-led."
"We insist on the territorial integrity of Syria, and the armed forces of Syria should control it," said Zarif.
Suspected regime forces personnel captured in Ras al Ayn
Meanwhile, Turkish forces detained 18 people, suspected of being part of Syria's regime forces, around the war-torn country's northern Ras al Ayn town, the defence ministry said on Tuesday.
The statement added that the issue was being coordinated with Russian officials.
"The 18 people who were claimed to be regime elements were captured alive in the southeast of Ras al Ayn during the search/detection/reconnaissance/security activities. The issue is being examined and coordinated with the authorities of the Russian Federation," the ministry said on Twitter.