British Prime Minister Theresa May has implored Saudi leaders to ease a blockade on war-torn Yemen to “avert a humanitarian catastrophe”, her office said on Thursday, echoing urgent appeals from the United Nations.

A Saudi-led coalition battling Houthi rebels has only partially lifted a crippling aid blockade on Yemen, which was imposed earlier this month in response to a missile fired by the Houthis that was intercepted near Riyadh airport.

May met both Saudi King Salman and powerful Crown Prince Mohammed bin Salman late on Wednesday on the second leg of her Middle East tour, with the worsening crisis in Yemen topping her agenda.

“The prime minister made clear that the flow of commercial supplies on which the country (Yemen) depends must be resumed if we are to avert a humanitarian catastrophe,” May's office said. “They agreed that steps needed to be taken as a matter of urgency to address this.”

Saudi Arabia is Britain's largest trading partner in the Middle East, and London has signed off on more than $4.4 billion worth of arms sales to Riyadh since March 2015.

That was the month that Riyadh launched its intervention against the Yemeni rebels who still control the capital Sanaa and much of the north of the country.

The war has since killed around 8,600 people, while a further 2,000 have died of cholera.

The United Nations on Monday urged the Saudi-led coalition to do “much more” to ease the blockade impeding shipments of aid and fully reopen the key rebel-held Red Sea port of Hodeida.

The coalition has allowed some supplies to reach rebel-held Sanaa and the Saleef Red Sea port, also in the Huthis' hands.

But little aid has entered through Hodeida, the main conduit for UN-supervised deliveries of food and medicine.

'Destabilising behaviour'

UN officials say Yemen could face the world's largest famine in decades unless the crippling blockade is lifted.

“The strong message I will be giving to Saudi Arabia... is that we want to see Hodeida port opened for humanitarian and commercial access,” May told the BBC before arriving in Riyadh.

During her visit, May also discussed Iran, Saudi Arabia's arch foe.

“The prime minister noted that we shared Saudi Arabia's concerns about Iran's destabilising regional behaviour,” her office said.

The Saudi-led coalition, which began its military intervention in Yemen in 2015, has accused Iran of supplying ballistic missiles to the rebels. Tehran denies the charge.

Saudi Arabia and Iran have a long-standing rivalry based as much in geostrategic interests as religious differences.

May arrived in Riyadh after a surprise visit to Baghdad where she met her Iraqi counterpart Haider al-Abadi and hailed recent military gains against the militant Islamic State group.

On Thursday the British leader will be in Jordan, a close British ally and partner in the US-led coalition fighting IS in Iraq and Syria.

She will meet with King Abdullah II and Prime Minister Hani Mulki on the final leg of her tour, which has been overshadowed by a high-profile row with US President Donald Trump over Islam.

On Tuesday, Trump drew fierce condemnation at home and abroad for retweeting three incendiary anti-Muslim videos posted by the deputy head of a British far-right group who has been convicted of a hate crime.

When challenged over the tweet, he then plunged deeper into controversy by suggesting May focus on defending Britain rather than criticising him.