Yemen may be on the brink of a new cholera epidemic, with a heightened death rate due to widespread malnutrition, the World Health Organisation has said.

The United Nations is hoping for a ceasefire in the north of the country to allow for vaccinations.

We’ve had two major waves of cholera epidemics in recent years and unfortunately the trend data that we've seen in the last days to weeks suggests that we may be on the cusp of the third major wave of cholera epidemics in Yemen," WHO emergency response chief Peter Salama told reporters in Geneva.

"We're calling on all parties to the conflict to act in accordance with international humanitarian law and to respect the request of the UN and international community for three full days of tranquillity and to lay down arms to allow us to vaccinate the civilian population for cholera."

Northern Yemen has never had an oral cholera vaccination campaign, but 3,000 health workers plan to vaccinate more than 500,000 people over the next three days in and around the city of Hudaydah, the country's main port and a key element in UN plans for a political solution to the war.

Yesterday, Saudi-led air strikes hit a fishing port and fish market in Hudaydah, which is held by Iran-aligned Houthi forces, and 55 people were killed, Yemeni medical sources said.

Dozens of others were wounded, the Red Cross said, denouncing the "reprehensible" disregard for human life.

"While the exact circumstances around the ground explosions are still unknown, this lack of respect for civilian life and civilian property is reprehensible," said Johannes Bruwer, head of the delegation in Yemen for the International Committee of the Red Cross, in a statement today.

Al-Thawra Hospital, Yemen's biggest, was caught in the attack.

Lise Grande, the UN humanitarian coordinator for Yemen, said in a statement that hundreds of thousands of people depended on the hospital to survive.

"Every day this week we have seen new cholera cases in Hudaydah, and now this.

"The impact of the strikes is appalling. Everything we are trying to do to stem the worlds worst cholera epidemic is at risk."

Mr Salama said the main hospital building remained intact but many subsidiary departments, especially the statistical department, were hit, affecting workers preparing for the cholera campaign.

This year's cholera incidence was not at the massive level seen a year ago, when case numbers surged to an eventual 1.1 million, but the steady recent increase pointed to a new outbreak beginning, he said.

Previous outbreaks might have helped build cholera immunity in the population, but other diseases, as well as malnutrition, have weakened Yemenis' immune systems.

"What we're likely to see is that interplay with cholera and malnutrition occurring more and more ... and not only more cases because of that but even higher death rates among the cholera cases that do occur, because people just don't have the physical resources to fight the disease any longer," Mr Salama said.