The UN's humanitarian chief said Tuesday he was "very alarmed" by a Huthi rebel advance on the Yemeni government's last northern stronghold, saying an assault on Marib could endanger millions of civilians. The Iran-backed Huthis have this month resumed an offensive to seize oil-rich Marib, some 120 kilometres (75 miles) east of the rebel-held capital Sanaa. The city's loss would be a major blow for Yemen's government, which is backed by a Saudi-led coalition, but also for the civilian population and the hundreds of thousands of displaced people sheltering in desolate camps in the region. "I'm very alarmed about the military escalation in Marib and its impact on the humanitarian situation," Mark Lowcock, the UN undersecretary general for humanitarian affairs, said in a tweet. "An assault on the city would put two million civilians at risk, with hundreds of thousands potentially forced to flee -- with unimaginable humanitarian consequences. Now is the time to de-escalate, not to add even more to the misery of the Yemeni people." Military officials told AFP that the rebels had advanced towards the city on two fronts overnight after heavy fighting with government forces.