“The Houthis’ assault on Marib is the action of a group not committed to peace or to ending the war afflicting the people of Yemen,” said the US State Department.

The United States urged Yemen’s Iran-backed Houthis on Tuesday to stop their advance on the government-held city of Marib and take part in international efforts to find a political solution to the violence in the Arab country.

“The United States urges the Houthis to halt their advance on Marib and cease all military operations and turn to negotiations,” the State Department said in a statement. “The Houthis’ assault on Marib is the action of a group not committed to peace or to ending the war afflicting the people of Yemen.”

The UN warned the offensive by Yemen’s Houthi group to take Marib city threatens to displace hundreds of thousands and complicate a renewed diplomatic push to end the war, UN officials say.

The gas-rich region of Marib has been a refuge for hundreds of thousands of people fleeing violence during Yemen’s six-year-old war, expanding its main city rapidly.

A frontline is now roughly 30km away to the city’s west, a government official told Reuters, and humanitarian concerns are growing.

“An assault on the city would put two million civilians at risk, with hundreds of thousands potentially forced to flee – with unimaginable humanitarian consequences,” U.N aid chief Mark Lowcock said on Tuesday, urging de-escalation.

The recent push towards Marib by Houthi forces, who control Yemen’s most populous areas, comes alongside intensified drone attacks into Saudi Arabia by the Iran-aligned group. Riyadh, which leads a coalition fighting the Houthis, said one attack started a fire in a civilian aircraft at an airport in southern Saudi Arabia on Wednesday.

Houthi offensives in recent days have hit camps for internally displaced persons (IDPs), causing people to flee, Yemen’s information minister said.

Najeeb Al-Saadi, director of the government’s IDP camp management unit said Houthi forces on Monday hit four camps to Marib city’s east. Two locations were totally evacuated, he said.

Residents in Marib city told Reuters on Tuesday they have heard the sound of explosions this week, more strongly at night, and that planes belonging to the Saudi-led coalition are a constant sight overhead.

The UN’s International Organisation for Migration (IOM)says 106,449 people have been displaced by fighting which has been bubbling along the Marib frontlines for a year.

It warned an additional 385,000 people could be displaced with a major frontline shift. There are 125 displacement sites around Marib, IOM says.