Houthi rebels must stop their assault on the government-held city of Marib in Yemen and take advantage of a peace process reinvigorated by US President Joe Biden, the UN's peace envoy to the country said on Thursday. Addressing online UN Security Council talks, UN envoy Martin Griffiths said the Houthi group’s “sharp escalatory turn” into the northern city was adding to the misery in an impoverished country on the brink of famine. “The attack on Marib must stop,” Mr Griffiths said. “It puts millions of civilians at risk, especially with the fighting reaching camps for internally displaced persons. The quest for territorial gain by force threatens the prospects of the peace process.” The council met against a backdrop of renewed fighting in Yemen's six-year civil war, with a rebel Houthi offensive on Marib, about 120 kilometres east of Yemen's rebel-held capital, Sanaa, the government's last northern stronghold. The loss of the city and nearby oil and gas fields would be a blow for the UN-recognised government of Yemeni President Abdrabu Mansur Hadi, which is backed by a Saudi-led military coalition. Mr Griffiths praised the "international momentum behind finding a peaceful resolution to the conflict" and the "renewed focus" under Mr Biden, who this month appointed Tim Lenderking as his envoy to Arabia's poorest nation. "International support for ending the conflict is indispensable, and offers a new opportunity to reopen space for a negotiated solution," said Mr Griffiths, who in recent days met Iranian, Saudi, Yemeni and American negotiators. All sides need to lay down their guns, return to the negotiating table and engage in more prisoner swaps and pave the way for an interim government in the run-up to national elections and an inclusive government, the UN envoy said. The advance by the Iran-backed Houthis has complicated US President Joe Biden’s efforts to help end Yemen’s war by bolstering peace talks. Mr Biden has cut military support for Washington’s regional ally, Saudi Arabia. The Biden administration also revoked the designation of the Houthis as a foreign terrorist group and lifted some sanctions against them in a reversal of an 11th-hour decision by the Trump administration. Mr Lenderking said he was “very aggressively” pursuing back-channel negotiations with the Houthis, a tribe from the mountainous north, and has urged Iran to stop training, supplying and arming the rebels. US State Department spokesman Ned Price this week urged the Houthis to halt the Marib attack, cease all military operations, end cross-border attacks on Saudi Arabia and engage with UN-led peace talks. UN aid chief Mark Lowcock said the assault is worsening the world's most serious humanitarian catastrophe – Marib governorate hosts an estimated one million people displaced from other areas by fighting. “Yemen is speeding towards the worst famine in decades,” Mr Lowcock, the undersecretary general for humanitarian affairs, told the council. Muhsin Siddiquey, Yemen country director for the charity Oxfam, told <em>The National</em> that the Marib offensive "risks catastrophe" because the city is home to 850,000 people who were forced to flee their homes during previous bouts of Yemen's war. Refugee settlements “are close to the front line and civilians have been caught in the offensive”, Mr Siddiquey said. “All sides of the conflict must agree to an immediate ceasefire to prevent further suffering in Marib and throughout the country.” Yemen’s grinding war has killed tens of thousands and displaced millions, according to international monitors, pushing the country towards famine and what the UN calls the world’s worst humanitarian crisis. Nearly 21 million Yemenis – more than two thirds of the population – survive on handouts. More than 12 million of them are in acute need and almost 50,000 are enduring “famine-like conditions”, the UN said on Wednesday. US envoy Richard Mills urged the Houthis to "immediately halt" their advance on Marib and the group's "destabilizing and inflammatory" attacks in Yemen and Saudi. Washington will back the renewal of UN sanctions and monitors, which is up for review this month, he added.<br/> "We call on the Houthis to immediately cease all attacks, including those that damage civilian infrastructure in Saudi Arabia, and to immediately halt the advance toward Marib, which is only bringing more suffering to the Yemeni people." UN aid chief Mark Lowcock warned that an emergency repair mission to the decaying FSO Safer oil tanker moored off a Houthi-held coastal area, which was originally set to take place in January, may not go ahead at all.<br/> "It is now difficult to say when exactly the mission might go," said Mr Lowcock.<br/> The Houthis "recently made several new requests that the UN can't meet. Mission preparations can't be finalized until these issues are also resolved", he added.<br/> The 45-year-old vessel has been stranded 8km southwest of Ras Issa oil terminal, 60km north of the Houthi-held port of Hodeidah, since 2015. The UN says it could rupture and spill 1.1 million barrels of oil into the Red Sea at any moment.