The final days of the year will end on a tragic note for Yemeni civilians, the UN envoy to the country, Hans Grundberg, said on Wednesday.

Yemen has been mired in war since Iran-backed Houthi rebels ousted the internationally recognised government from the capital Sanaa in 2014.

“2021 is ending on a tragic note for Yemenis, millions of whom are struggling with poverty, hunger and severe restrictions on their freedom of movement,” Mr Grundberg said.

The UN official called for Sanaa airport to be opened and for the removal of obstacles hindering Yemenis’ ability to move within or between governorates in Yemen.

Since 2014, the airport has been controlled by the Houthis. They said the airport had been put out of operation after air strikes by the Saudi-led coalition.

Since the fighting began in 2015, the airport has been closed to civilian flights, although UN planes have been permitted to land there.

On Monday, the rebels said they allowed the resumption of UN flights to Sanaa airport "after malfunctions in communications and navigational devices were temporarily fixed", the agency report said.

They also urged the UN to help facilitate the delivery of air traffic control equipment from Djibouti.

Mr Grundberg "denounced the recent major military escalation in Yemen and called on the warring parties to immediately de-escalate".

“The escalation in recent weeks is among the worst we have seen in Yemen for years and the threat to civilian lives is increasing," he said.

The continued offensive on Marib and missile attacks on the governorate are "resulting in civilian casualties, damage to civilian objects and mass displacement”, Mr Grundberg said.

The battle for Marib began in February and has intensified in recent weeks, with Houthi gains across the gas and oil-producing region, and increased air strikes by the Saudi Arabia-led coalition to deter them.

The UN envoy expressed his concern about "sustained attacks against Saudi Arabia, which also resulted in civilian casualties and damaged civilian infrastructure".

In recent weeks, attacks against the kingdom by the rebels have increased. On Saturday, an attack on the Saudi border town of Jizan killed two civilians and injured seven.

The war in Yemen has created what the UN called the world’s worst humanitarian crisis.

More than half of Yemen’s population of 16.2 million people faces acute hunger, with 2.3 million children at risk of malnutrition, the UN food agency estimates.