U.N. Special Envoy for Syria Geir Pedersen has called on all armed groups in Syria to uphold law and order while preserving public institutions.
Speaking in Doha on Sunday, Pedersen also added that he had no information on the whereabouts of Bashar Assad.
In an earlier statement, he called the fall of the capital Damascus anti-regime forces "a watershed moment."
Describing Syria's drawn-out civil war as a "dark chapter (that) has left deep scars," special envoy Pedersen said: "Today we look forward with cautious hope to the opening of a new one – one of peace, reconciliation, dignity and inclusion for all Syrians."
"Today marks a watershed moment in Syria's history – a nation that has endured nearly 14 years of relentless suffering and unspeakable loss," he said, extending his "deepest solidarity to all who have borne the weight of death, destruction, detention and untold human rights violations."
Bashar Assad's reported flight from Damascus comes less than two weeks after an alliance of anti-regime forces, led by the Hayat Tahrir al-Sham (HTS), launched a lightning offensive challenging more than five decades of domination by the Assad family.
Assad had ruled Syria since 2000, when he inherited power from his father.
Since 2011, he had overseen a crackdown on the democracy movement, which began with peaceful protests.
The Assad government's repression of the protests morphed into one of the bloodiest wars of the century, killing hundreds of thousands of people and displacing millions.
Pedersen acknowledged in a statement that "the challenges ahead remain immense," stressing however: "This is a moment to embrace the possibility of renewal ... (and) a path toward a united and peaceful Syria."
The U.N. special envoy emphasized "the clear desire expressed by millions of Syrians that stable and inclusive transitional arrangements are put in place, that Syrian institutions continue to function."
He said the Syrian people should be helped to realize their "legitimate aspirations ... (to) restore a unified Syria."
The statement said Pedersen had heard from many Syrians, including armed groups and civil society organizations and had noted "a will to protect their fellow Syrians ... against retribution and harm."
He appealed to all armed parties to "maintain ... law and order, protect civilians and preserve public institutions."
He urged Syrians "to prioritize dialogue, unity and respect for international humanitarian law and human rights as they seek to rebuild their society".
"There must be a collective effort to secure peace and dignity for all."
