A pro-government Yemeni soldier fires a machine gun on June 7, 2018 near the town of Al Jah southeast of Hodeida (Photo: AFP)

Yemeni forces backed by a Saudi-led coalition launched a major offensive Wednesday to retake the rebel-held port city of Hodeida, a key aid hub, sparking calls from the United Nations for restraint.

Pro-government troops began the assault despite mounting international fears about the humanitarian fallout, pressing toward Hodeida airport south of the city after receiving a "green light" from the coalition.

The Red Sea port, controlled by the Iran-backed Huthi rebels who hail from northern Yemen, serves as the entry point for 70 percent of the impoverished country's imports as it teeters on the brink of famine.

The UN envoy for Yemen, Martin Griffiths, said he was continuing to hold negotiations on keeping the key port open to aid deliveries.

"We are in constant contact with all the parties involved to negotiate arrangements for Hodeida that would address political, humanitarian, security concerns of all concerned parties," he said in a statement.

Hodeida residents were in a state of anxiety, waiting for the fighting to reach their neighbourhoods. Those contacted by AFP said Huthi fighters had fanned out across the city.

Coalition sources said the alliance carried out 18 air strikes on Huthi positions on the outskirts of Hodeida on Wednesday.

According to medical sources in the province, 22 Huthi fighters were killed by coalition raids, while three pro-government fighters were killed in a rebel ambush south of Hodeida.

The city, home to 600,000 people, was captured by the insurgents in 2014 along with the capital Sanaa.

Saudi Arabia, the United Arab Emirates and a bloc of other countries intervened in Yemen the following year with the goal of restoring the government to power.

The coalition accuses the Huthis of using the port to secure Iranian arms, notably the ballistic missiles the militants have increasingly fired into Saudi territory.

Yemen's government said on Tuesday that negotiations had failed to force the rebels from Hodeida, and that a grace period for UN-led peace efforts was over.

"All peaceful and political means of removing the Huthi militia from Hodeida port have been exhausted," the government said in a statement carried by Yemen's state news agency Saba.

The United Nations on Monday withdrew its international staff from Hodeida.

The UAE, a pillar of the anti-Huthi coalition, says crucial aid will not be halted by the offensive.

"Hodeida port remains open to shipping," Emirati State Minister for Foreign Affairs Anwar Gargash tweeted Wednesday.

"We have also put contingency plans in place to move aid by other methods to Hodeida and points beyond," he said.

The high-ranking UAE diplomat insisted that similar large-scale offensives had improved the lives of Yemenis in Aden and elsewhere.

The International Committee of the Red Cross warned however that any battle for Hodeida would exacerbate the suffering of the population, already "weakened to extreme levels".

"Life-saving items cannot be given to those in need while fighting is ongoing," ICRC Middle East director Robert Mardini said in a statement.

"Lifelines to the outside world must be maintained."

Yemeni forces massing around Hodeida are a mix of local fighters, those loyal to President Abedrabbo Mansour Hadi, and supporters of the ex-head of state, Ali Abdullah Saleh.

They are backed on the ground by the UAE, while Saudi Arabia has been leading a campaign of air strikes.

Analysts say anti-rebel forces are determined to drive the Huthis from the key port, having failed to score any major victories since the first year of the war.

The UN Children's Fund (UNICEF) has raised alarm over the plight of Hodeida's 300,000 children and the risk that drinking water supplies will be disrupted.

"UNICEF has pre-prepositioned supplies in Hodeida: over 20,000 basic hygiene kits (one kit per family). 40,000 additional kits are in the pipeline. We hope we don't need to use them," UNICEF Yemen representative Meritxell Relano said on Twitter.

The Huthi leadership on Tuesday called on the international community to "pressure a halt to the escalation", warning an assault on Hodeida would put Red Sea navigation at risk.

On Wednesday, the Huthis said they targeted a coalition warship off the coast of Hodeida with two missiles, with rebel outlet Al-Masirah claiming a direct hit.

As the offensive got under way, Saudi Arabia's ambassador to Yemen was defiant.

"Hodeida will be liberated, and the Yemeni people will gain back a major artery of life. An essential lifeline that was previously plagued by the Iranian backed Huthi militia," he tweeted.