Sixty-five combatants have been killed after Yemen's Houthi rebels launched a renewed offensive on Marib, the last government stronghold in the country's oil-rich north, a military official said on Thursday.

The clashes erupted when the Iran-backed rebels attacked pro-government positions south of the city, making progress despite losing dozens of fighters in coalition airstrikes. It was the Houthis' first major offensive on Marib, the key to controlling the region, since June, when 111 fighters on both sides died in three days of clashes.

"Twenty-two pro-government (forces) were killed and 50 others were wounded, while 43 Houthi rebels were also killed in the last 48 hours," a government military official said, in figures that were confirmed by other military and medical sources.

The fighting comes after strikes on Yemen's largest air base, in the country's south, killed at least 30 pro-government fighters on Sunday in the deadliest incident since December.

Yemen's internationally recognized government – backed by a Saudi-led military coalition – and the Houthi have been locked in war since 2014, when the insurgents seized the capital Sanaa.

In February, the Houthi escalated their efforts to seize Marib, the government's last northern stronghold in fighting that has killed hundreds on both sides. Control of the oil-rich region would strengthen the Houthis' bargaining position in peace talks.

While the U.N. and Washington are pushing for an end to the war, the Houthis have demanded the reopening of Sanaa airport, closed under a Saudi blockade since 2016, before any cease-fire or negotiations. As well as the bloody offensive in Marib, the Houthis have also stepped up drone and missile strikes on Saudi targets, including its oil facilities.

In June, the former U.N. envoy for Yemen, Martin Griffiths, told the Security Council that his efforts over the past three years to end the war had been "in vain".

The fighting has killed tens of thousands and left some 80% of Yemenis dependent on aid, in what the U.N. calls the world's worst humanitarian crisis. The war has also displaced millions of people and left many on the brink of famine.