Geir Pedersen, the UN's special envoy for Syria, also called at the Security Council for Israel to "cease all settlement activity in the occupied Syrian Golan" and said an end to sanctions would be key to assisting Syria.
"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," he said.
"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.
'Attacks on Syria's sovereignty'
"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.
Pedersen noted Israel had conducted more than 350 strikes on Syria following the departure of the former regime, including a major strike on Tartous.
"Such attacks place a battered civilian population at further risk and undermine the prospects of an orderly political transition," he said.
The envoy warned against plans announced by Israel's cabinet to expand settlements inside the Golan, occupied by Israel since 1967 and annexed in 1981.
On Tuesday, Israeli Prime Minister Benjamin Netanyahu held a security briefing atop a strategic Syrian peak inside the UN-patrolled buffer zone on the Golan Heights that Israel seized this month.
"Israel must cease all settlement activity in the occupied Syrian Golan, which are illegal. Attacks on Syria's sovereignty and territorial integrity must stop," said Pedersen.
