Yemen's seven-year war lurched into its latest crisis Monday with Huthi rebels saying they have seized an Emirati-flagged military ship which the Saudi-led coalition insisted was carrying medical supplies.

The Iran-backed Huthis released images of what they said were military jeeps and weapons on board the vessel, named Rwabee, which was captured in the Red Sea off the Yemen coast.

The coalition branded the hijacking an act of piracy and threatened to seize the ship back by force. The Rwabee was returning from a mission to set up a field hospital on Yemen's Socotra island, it said.

"The militia must promptly release the ship, or the coalition forces will undertake all necessary measures and procedures to handle this violation, including the use of force if necessary," coalition spokesman Turki al-Malki said.

But the rebels, who said the multinational crew was still on board, warned there would be consequences if the ship was targeted.

The incident follows intensified fighting in recent weeks including multiplying Saudi airstrikes on rebel territory, while the Huthis have ramped up missile and drone assaults on the neighbouring kingdom.

The coalition has been fighting for nearly seven years in support of Yemen's government against the Huthis, in a conflict that has displaced millions and created the world's worst humanitarian crisis, according to the United Nations.

There was no immediate comment from the United Arab Emirates.

A US official based in the region said it appeared to be the first ship hijacking by the Huthis since they seized three in late 2019.

"The seizure was unusual... It looks to be the first known case of the Huthis seizing a Saudi-led coalition vessel in more than two years," the official said, on condition of anonymity.

Displaced Yemenis return to their tents with aid donated by a Kuwaiti charitable organisation on the outskirts of Marib, the government's last northern stronghold
Displaced Yemenis return to their tents with aid donated by a Kuwaiti charitable organisation on the outskirts of Marib, the government's last northern stronghold AFP / STR

Malki, cited by the official Saudi Press Agency, said the Rwabee was returning to the Saudi city of Jazan carrying medical supplies when it was seized late on Sunday off Hodeida province.

The Huthis' military spokesman Yahya Saree said the ship had "entered Yemeni waters without authorisation" and was carrying out "hostile acts".

He later held a news conference where he showed video footage purportedly of military hardware on the Rwabee.

"The successful and unprecedented operation is part of the fight against (coalition) aggression," Saree tweeted.

The incident echoes November 2019, when the Huthis seized two South Korean vessels and a Saudi-flagged tug in the Red Sea a few miles off Uqban island, north of Hodeida.

In July 2018 the kingdom accused the rebels of attacking two Saudi-operated oil tankers in the Red Sea. The pro-Huthi Al-Masirah television said that the rebels had targeted a Saudi warship named Al-Dammam, without providing further details.

The rebels have been engaged in a ferocious assault on Marib, the government's last northern stronghold, and in November they took control of a large area south of the lifeline port of Hodeida.

On December 25 the coalition launched a "large-scale" assault on Yemen after missiles fired by the rebels killed two people in the kingdom, the first such deaths in three years.

Analyst Stephane Lacroix of Paris's Sciences Po university said the Huthis were in a "relatively strong position, unwilling to negotiate and trying to get the Saudi coalition to basically surrender".

"The Saudis... have launched a major offensive to put as much pressure as possible on the Huthis to try to force them into a compromise. For the moment, that hasn't been working," he told AFP.

"This is just the Huthis increasing the pressure in return showing that they aren't willing to back down. And I don't think the Saudi coalition can do much more than what it is already doing at this point."

Efforts by the United Nations and United States to end the conflict have so far made little progress.

In late November the United Nations said the war would have killed 377,000 people by years' end, both directly and indirectly through hunger and disease.