The Saudi-led coalition's easing of its blockade on famine-threatened Yemen is "a step in the right direction" but does not go far enough, the European Union and United Nations said.

The coalition shut down Yemen's borders on Monday in response to a missile attack by Houthi rebels that was intercepted near Riyadh airport.

On Wednesday, it reopened the southern port of Aden, controlled by coalition-backed government forces, and the Wadea crossing on the Saudi-Yemeni border was reopened the next day.

The EU's commissioner for humanitarian aid and crisis management, Christos Stylianides, said Saturday that more steps were needed.

"Initial measures to resume operations in Aden port and to open Wadea border crossing are a step in the right direction," he said.

"The EU urges the coalition to ensure the immediate resumption of the UN's flights and activities in the ports of Hodeida and Saleef and the opening of land borders for humanitarian relief and basic commercial commodities.

"The delivery of life-saving supplies is critical for the Yemeni population and must be facilitated by all parties to the conflict," Stylianides added.

On Friday, the UN office for the coordination of humanitarian aid, OCHA, said the coalition was still blocking desperately needed UN aid deliveries to Yemen, despite the reopening of Aden and Wadea.

"Humanitarian movements into Yemen remain blocked," said OCHA spokesman Russell Geekie.

"The reopening of the port in Aden is not enough. We need to see the blockade of all the ports lifted, especially Hodeida, for both humanitarian and for commercial imports."

'Famine'

UN aid chief Mark Lowcock told the Security Council this week that unless the blockade is lifted, Yemen will face "the largest famine the world has seen for many decades, with millions of victims".

Stylianides echoed Lowcock's concerns.

Yemen "is suffering the world's worst humanitarian crisis, with more than two-thirds of its population in need of humanitarian assistance", he said in a statement.

"The EU shares the concerns expressed by... Lowcock and calls for full and unrestrained access to be restored immediately, to avoid Yemen suffering the largest famine in decades," Stylianides said.

Geekie said no aid had gone into Aden yet and the reopening of the Wadea crossing did not affect UN operations.

The transport minister of Yemen's internationally recognised government, Murad al-Halimi, said two airports in Aden and the loyalist-held southern city of Seiyun would reopen from Sunday.

Yemeni media quoted him as saying the national carrier, Yemenia, would resume its flights to and from the two airports to destinations including Amman and Cairo.

The sea port at Hodeida, which is in rebel-held territory, is key to UN aid efforts as it is closest to the majority of people in need.

The coalition accuses rebels of using aid shipments to smuggle in weapons.

Before the blockade, UN aid agencies were delivering food and medicine through Hodeida, Saleef and Aden ports.

"There can be no alternative for all these ports being fully functional and receiving commercial and humanitarian cargo," Geekie said.

The UN has listed Yemen as the world's number one humanitarian crisis, with 17 million people in need of food, seven million of whom are at risk of famine.

More than 2,000 Yemenis have died in a cholera outbreak now affecting nearly one million people.

Saudi Arabia and its allies intervened in neighbouring Yemen in March 2015 to push back the Iran-backed Houthi rebels who control the capital, Sanaa, and restore the government of President Abd-Rabbu Mansour Hadi to power.