A Houthi rebel attack on the Yemeni government's last northern stronghold must stop, the UN envoy to the country said Thursday, warning of a potential humanitarian disaster.

"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," said Griffiths.