Merchant vessels are taking weeks to deliver vital food supplies to Yemen as Saudi-led coalition warships search for arms bound for Iran-allied Houthi fighters and heavy fighting disrupts shipments in a worsening humanitarian crisis.

The conflict has hurt imports to Yemen, where about 20 million people or 80 per cent of the population, are estimated to be going hungry.

The Arabian peninsula's poorest country, Yemen imports more than 90 per cent of its food, including most of its wheat and all its rice - most of it by sea. It faces increasing problems as many shipping companies have pulled out and those still willing to bring cargoes in face a long wait to get navy clearance.

The prospect of a humanitarian emergency has added to international pressure on Saudi Arabia to bring its military campaign to a close.

Riyadh is leading a coalition of Sunni Muslim states in support of exiled president Abd-Rabbu Mansour Al-Hadi against the Houthis, fighters from the Shi'ite minority, who have emerged as the country's most powerful force since seizing the capital last year.

Before the fighting, entering and discharging at Yemen's ports would have taken a few days at most. Picking up deliveries from the ports is also taking longer now as logistical networks are increasingly strained by the violence and fuel shortages.

At least 10 merchant ships carrying wheat and other food commodities are still waiting offshore to discharge at Yemen's functioning ports of Salif and Hodaida on the Red Sea - some of them stuck for at least two weeks, according to ship and port tracking data, and confirmed by industry sources.

Three vessels carrying corn, rice and other foodstuffs were currently discharging in Hodaida and Salif - a process that took weeks for some, tracking data showed. While two other ships - both carrying wheat - had waited for over 10 days before being diverted to other destinations, data showed.

"It will remain slow and complex to bring ships into Yemen for some time to come," an international commodities trade source said.

"There is no timeframe for how long you can wait before getting clearance, and the fighting inside Yemen is getting worse ever day. On top of that there are payment hiccups. This is a high risk trade."

Trade sources said Hodaida port was operating at a slower capacity as many foreign workers had fled, which was hampering operations.

Last week airstrikes by Saudi-led coalition jets on Houthi targets came close to the port of Salif for a second time in days. One cargo vessel, the Lycavitos, carrying 47,250 tonnes of wheat, was discharging part of its cargo in Salif at the time and "felt the tremors" from the sorties, which were 4 km away, the ship owner's agent said.

"The stevedores fled the port area but we were able to complete discharging with no damage to the vessel," Helikon Shipping Enterprises said.

Helikon said the Lycavitos was still waiting for clearance to dock at Hodaida to deliver the rest of the cargo along with nine other vessels, which were drifting in sea lanes off the coast.

"There is no guidance or timeline given by the coalition naval forces regards when clearance might be given," Helikon said.

Trade sources said another vessel carrying wheat took two weeks to get approval to part discharge at Salif and then had to undergo another inspection to be able to unload its remaining cargo at Hodaida.

"Armed forces went on board to check thoroughly and it was given permission to sail for Hodaida at 2105 hours on April 30. But that was revoked within one hour at 2200 hours by the same warship," another trade source said.

"Even after explaining that this vessel is only carrying wheat, the coalition forces clearly refused the vessel to sail to Hodaida. The vessel is still waiting for approval."

Saudi Arabia on Thursday proposed a five-day humanitarian truce in Yemen, but said a ceasefire depended on the Houthi militia and its allies also agreeing to lay down arms.

US Secretary of State John Kerry welcomed the proposal and added that neither Saudi Arabia nor the United States was talking about sending ground troops into Yemen.

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Abdelbari Taher, a Yemeni writer and resident in the capital Sanaa, said life for ordinary people was deteriorating.

"The situation in Yemen is catastrophic, especially in light of the lack of fuel products. There are no bakeries, traffic is grinding to a halt and water services are stopping," he told Reuters.

"The (Saudi) blockade was supposed to be used to prevent arms from reaching Yemen. But instead, everything is being prevented from entering, including humanitarian supplies. There is increasing discontent among Yemenis."

Trade sources looking to bring in food said the situation was unlikely to change for now.

"The logistical problems are not going to get any easier at the moment given also the lack of fuel and frequent power shortages," the commodities trade source said.

A shortage of fuel has crippled hospitals and food supplies in the past weeks, and the UN's World Food Programme has said its monthly fuel needs have leapt from 40,000 litres a month to one million litres. - Reuters