The United Nations Thursday appealed to all sides in the Yemen conflict to respect a fragile five-day truce in the country in a bid to boost the delivery of sorely-needed aid.

The plea made by the UN envoy to Yemen, Ismail Ould Sheikh Ahmed, came as Saudi-led forces accused Shia rebels of violating the truce on Thursday, two days after it took effect.

But the Saudi-led coalition said it would abide by the ceasefire, and stick to its decision to temporarily halt weeks of air strikes on Iran-backed Houthi rebels.

The UN envoy said in a statement he was "very worried about violations of the truce", urging all sides to "strictly respect a cessation of military operations... to allow the flow of aid".

Wrapping up his first visit to Yemen, he urged all sides involved in the conflict to spare airports, ports and any other infrastructure necessary for the smooth delivery of aid to the embattled population.

The humanitarian pause that began late Tuesday is the first break in the air war the coalition launched on March 26 in support of exiled President Abedrabbo Mansour Hadi, and has backing from Washington.

A coalition statement said Houthi rebels had violated the truce 12 times, including with artillery and rocket attacks in several towns in southern Yemen.

Despite the alleged violations, the coalition pledged "its full commitment to the humanitarian truce and restraint".

Residents said calm prevailed across most of the country except in the cities of Taez, Daleh and oil-rich Marib, where they reported intermittent exchanges of fire between rebel and pro-Hadi forces.

The Houthis and their allies have pledged to abide by the ceasefire while Saudi Arabia has warned it will punish any attempt to exploit the truce.

Washington backs the truce and a US State Department spokesman has said it was largely holding, noting however reports of some clashes.

The Saud-led campaign was launched to reinstate the rule of Hadi after the Houthis, backed by army units loyal to ex-president Ali Abdullah Saleh, overran much of Yemen.

More than 1,500 people have been killed since March in air strikes and fighting between rebel forces and Hadi loyalists, according to the United Nations.

An aviation official said operations at Sanaa airport, which was targeted in air raids, were "gradually returning" to normal after a plane arrived from Jordan on Wednesday with 150 passengers on board.

Another airport official said that two flights, from Doctors Without Borders and the United Nations, landed Thursday in Sanaa.

Warnings of a dire humanitarian crisis have mounted since the air war started, with aid agencies saying fuel, water and medicine were running out.

Nadia Sakkaf, Yemen's information minister now based in Riyadh, said seven vessels carrying food supplies, medical aid and fuel had docked in Yemeni ports.

She said daily flights linking Yemen to Jordan and Egypt would continue until May 18 -- when the truce is due to end.

Qatar and Kuwait said they will offer Yemen 120 and 40 tonnes of medical aid, respectively.

Saudi Arabia has offered its impoverished neighbour $540 million in aid and humanitarian operations.

Iran, which Saudi Arabia accuses of arming the rebels, has also said it is sending an aid ship to Yemen, prompting warnings from the government-in-exile.

Yemen's Foreign Minister Riyadh Yassin said from Saudi Arabia "all measures will be taken against the Iranian ship if it enters Yemeni territorial waters without permission" from the coalition.

On Thursday the government-in-exile announced the recall of the head of its embassy in Iran, accusing Tehran of "interference" in Yemen and "support for the Houthis".

The move came after Iran, which has dismissed as "utter lies" accusations that it armed the rebels, said it had coordinated with the United Nations for the docking of an aid ship in Yemen.

The Pentagon has called on Iran to change course and head instead for a UN aid hub in Djibouti.

But Iranian Deputy Foreign Minister Hossein Amir-Abdollahian insisted that Tehran had taken measures for the ship to travel to Yemen.

"The required coordination has been done with relevant authorities in the UN for the docking of the ship carrying Iran's humanitarian aid for Yemen," he said.