"There have been significant hostilities in the last two weeks, before a ceasefire was brokered... A five-day ceasefire has now expired and I am seriously concerned about reports of military escalation," said Geir Pedersen, the UN's special envoy for Syria.
"Such an escalation could be catastrophic."
Pedersen also said he had met with Syria's new de facto leadership following the rebels' lightning takeover, and toured Sednaya prison's "dungeons" and "torture and execution chambers," operated under Assad's government.
He called for "broad support" for Syria and an end to sanctions to allow for reconstruction of the war-ravaged country.
"Concrete movement on an inclusive political transition will be key in ensuring Syria receives the economic support it needs," Pedersen said.
"There is a clear international willingness to engage. The needs are immense and could only be addressed with broad support, including a smooth end to sanctions, appropriate action on designations, too, and full reconstruction."
Western countries are wrestling with their approach to Hayat Tahrir al-Sham (HTS), which spearheaded the takeover of Damascus, and has roots in the Syrian branch of Al-Qaeda. It has largely been designated in the West as a "terrorist" group, despite moderating its rhetoric.
