The humanitarian crisis in Yemen continues to deepen as more than 20 million people, or two-thirds of the population, need assistance, said Ramesh Rajasingham, UN assistant secretary-general for humanitarian affairs, on Thursday.

The most immediate cause of the worsening humanitarian situation is the ongoing war. In recent weeks, conflict has intensified. Yet a cease-fire - and all the benefits this would bring - remains elusive, Rajasingham told the UN Security Council in a briefing.

Meanwhile, Yemen's economy is collapsing, leaving millions destitute. This collapse is a direct result of the war and, more indirectly, a result of the perverse incentives the war has created, he said.

Hemmed in by violence and with fewer chances to take care of themselves, millions of Yemenis are looking to humanitarian organizations for help. But aid work is about to get much harder in Yemen, mostly because agencies are, once again, starting to run out of money, he said.

Over the last few weeks, Houthi forces have intensified their brutal offensive in Marib, taking more territory there and in neighboring parts of Shabwah governorate.

In Aden, clashes also erupted earlier this month between rival armed groups, leading to several days of violence that shut down parts of the city and raised grave concerns for the future.

Fighting, shelling and airstrikes have also continued in Saada, Hajjah, Hodeidah governorates, and along nearly 50 other front lines.

The impact on civilians is grim. Hostilities in September killed or injured 235 civilians across the country, Rajasingham said.

The fighting in Marib is taking a particularly heavy civilian toll. Last month, almost 10,000 people were displaced in Marib, the single highest figure so far this year.

Since late September, Houthi forces have encircled Al Abdiyah district in Marib, which is home to nearly 35,000 civilians, many of whom previously fled there to escape fighting in other areas, he said.