Houthi attack on Yemen government stronghold 'must stop': UN
"It puts millions of civilians at risk, especially with the fighting reaching camps for internally displaced persons," envoy Martin Griffiths told the UN Security Council by video conference, of the attack on the city of Marib.
"The quest for territorial gain by force threatens the prospects of the peace process."
The city's loss would be a major blow for Yemen's government, which is backed by a Saudi-led coalition, but would also threaten catastrophe for civilians, including hundreds of thousands of displaced people sheltering in desolate camps in the region.
"To seize this chance to revitalise the political process, the parties should immediately agree to a nationwide ceasefire that halts all forms of fighting," Griffiths said.
Tim Lenderking, newly named as the US special envoy for Yemen as part of President Joe Biden's push to end the war, this week also urged the Houthis to halt their advance.
The assault could "push an already stretched humanitarian infrastructure beyond the breaking point," Lenderking told reporters in Washington after a trip to the region.
Read more: Biden and the Saudi quagmire in Yemen
The toll from the battle for Marib is unknown, but there are reports of hundreds dead.
Until early 2020, Marib had been spared the worst of Yemen's six-year-old conflict, and became a sanctuary for many.