British Prime Minister Theresa May has called on Saudi Arabia to ease the siege on Yemen "as a matter of urgency" to avert "a humanitarian catastrophe".

The statement, published by May's office on Thursday, said the British leader met King Salman and Crown Prince Mohammed bin Salman late on Wednesday during her three-day tour of the Middle East.

"They discussed Yemen, where the prime minister made clear that the flow of commercial supplies on which the country depends must be resumed if we are to avert a humanitarian catastrophe," the statement said.

"They agreed that steps needed to be taken as a matter of urgency to address this and that they would take forward more detailed discussions on how this could be achieved."

'Famine-like conditions'

The 22 humanitarian groups issued a warning in early November that Yemen had only six weeks of food air remaining for about seven million Yemenis who were facing "famine-like conditions".

Saudi Arabia, which has been at war with Yemen since 2015, tightened its air, land and sea blockade of the country on Sunday, after Houthi rebels fired a ballistic missile towards the Saudi capital, Riyadh.

The UK has been criticised for selling arms used by the Saudi-led coalition.

The conflict that has seen more than 10,000 people killed and three million people displaced since 2015.