The United Nations called on Yemen's warring parties on Wednesday to safeguard the strategic Red Sea port of Hodeidah as a lifeline for millions of Yemenis facing potential famine.

The Yemeni government and its Arab allies are preparing an assault on Hodeidah port, which has been the entry of nearly 80 percent of Yemen's food imports, because they say the Iran-aligned Houthis use it to smuggle weapons and ammunition.

Local officials say the government and its allies have positioned two recently-trained brigades for a possible attack. One is 230 km (140 miles) north of Hodeidah and the other 130 km (80 miles) to the south, so they would have to cross large areas of Houthi-held territory if they set off to seize the port.

"The continued military escalation in Yemen, specifically the militarization of large regions on its Western Coast and the associated increase of humanitarian access obstacles and population movement restrictions, are of grave concern to the humanitarian community," the Office of the Humanitarian Coordinator in Yemen said in a statement issued on Tuesday.

"This is only resulting in more displacement, more institutional collapse, and more suffering."

More than two years of civil war have cut food deliveries by more than half and pushed the Arabian Peninsula's poorest country to the edge of famine. The United Nations says nearly 3.3 million people, including 2.1 million children, are acutely malnourished.

"The Yemen Humanitarian Country Team calls on all warring parties and on those with influence over the parties to ensure the continued functioning of Hodeidah Port," the statement said.

Yemen has historically imported 80 to 90 per cent of its food, mostly through Hodeidah. Five cranes at the port there have been destroyed by airstrikes, forcing dozens of ships to line up offshore because they cannot be unloaded.

"The port is located in a densely populated urban center where thousands of people live and any military campaign in its vicinity, from the ground or air, would have devastating civilian consequences," the agency warned.

The conflict pits the armed Houthi group against the government of President Abd-Rabbu Mansour Hadi, backed by a Saudi-led Arab alliance. More than 10,000 people have been killed by coalition air strikes and fighting on the ground.

The coalition was formed in 2015 to fight the Houthis and troops loyal to former president Ali Abdullah Saleh, who have fired missiles into neighboring Saudi Arabia.