"Smashed cranes" at Yemen's rebel-held Hodeida port are hindering the entry of relief supplies to ease a deteriorating humanitarian crisis in the war-torn country, the UN aid chief said Wednesday.

On a visit to Saudi Arabia, Stephen O'Brien told reporters that aid flow needed to increase at the Red Sea port, through which 80 to 90 percent of Yemen's supplies transited before the war.

"The real issue is the restriction of unloading capacity at the port because the cranes are smashed," said O'Brien, who heads the UN Office for the Coordination of Humanitarian Affairs (OCHA).

He did not mention the cause of the damage to the cranes, but in August last year he said coalition air strikes on the port were in contravention of international humanitarian law.

The United States and the European Union at the time expressed concern over the bombing, which Washington said hit "critical infrastructure" at the port.

Saudi Arabia has for 18 months led an Arab military coalition supporting Yemen's internationally recognised government against the Huthi rebels who seized much of the country.

The war has killed more than 6,600 people, displaced 3.15 million and left about a quarter of Yemen's population "not sure where the next meal is coming from", O'Brien said after talks with Saudi officials.

His remarks came a day after he said it was "a matter of urgency" that flows of food, medicine and fuel increase into Yemen.

O'Brien said efforts were being made to find "a better unloading capacity, as well as make sure there are no administrative burdens which are slowing the process".

The Saudi-led coalition has imposed a sea blockade on Yemen to prevent weapons reaching the rebels who it says are backed by Iran.

But O'Brien said a UN inspection mechanism to check commercial ships bringing supplies was working well, "with many ships now cleared to come into Hodeida".

- Averting famine -

Aid into Yemen via air was however limited, O'Brien said.

The coalition in August said it would allow humanitarian flights into Sanaa's international airport after several days of closure due to hostilities around the capital.

O'Brien -- who arrived in the kingdom from Yemen -- said he saw the impact of malnutrition in Hodeida.

There were "many children some of whom we thought were babies, (but) were in fact seven or eight years old".

He said the United Nations and its partners were providing assistance, every two months, to six out of the seven million people in Yemen most in need of food so as to avert famine.

In Riyadh, O'Brien met Yemen's President Abedrabbo Mansour Hadi "to discuss the deteriorating humanitarian crisis in Yemen and the need to ensure the protection of civilians as hostilities sadly continue and even intensify".

Some of the heaviest combat is around Yemen's third city of Taez, which is almost completely surrounded by the rebels, but loyalist forces have opened a road on the western side of the city, allowing aid to be brought in.

"Despite people suggesting otherwise, a very large quantity of aid is reaching Taez, both in the past and indeed as we speak this evening," O'Brien said.

He met the head of a Saudi government aid agency, the King Salman Humanitarian Aid and Relief Centre, to discuss "how our two entities can better work together in a more complementary and efficient way".

Riyadh faces mounting international scrutiny over civilian casualties in its Yemen air campaign.