"I call for urgent and serious political engagement, among Syrian and international stakeholders, to spare bloodshed and focus on a political solution in accordance with Security Council Resolution 2254" agreed in 2015, Geir Pedersen said in a statement.

"The latest developments pose severe risks to civilians and have serious implications for regional and international peace and security."

The Syrian army, supported by Russian air power, in 2016 had recaptured rebel-held areas of Aleppo, a city dominated by its landmark citadel.

But since Wednesday, an Islamist militant alliance has pressed a lightning offensive against the forces of the Syrian government.

The Islamist militant group Hayat Tahrir al-Sham and allied rebel factions "control Aleppo city, except the neighbourhoods controlled by the Kurdish forces," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights war monitor.

Pointing to broader challenges, Pedersen said: "What we see in Syria today is a mark of a collective failure to bring about what has plainly been required now for many years, a genuine political process to implement Security Council Resolution 2254".

The UN resolution adopted in 2015 outlined a roadmap for a political transition in Syria, including a nationwide ceasefire, drafting a new constitution and holding UN-supervised elections.

"I have repeatedly warned of the risks of escalation in Syria... and the reality that no Syrian party or existing grouping of actors can resolve the Syrian conflict via military means," the envoy said.