Yemeni forces backed by Saudi Arabia and allied Arab militias on Wednesday launched a military offensive to regain control of a Red Sea port-city that acts as an entrance point for Houthi rebel supplies and humanitarian aid.

At least 10,000 soldiers from Yemen, Sudan, Saudi Arabia and the United Arab Emirates took positions outside the city of Hudaydah before launching an assault on the airport, which is located in the southern outskirts of the settlement.

The United Nations warned that some 600,000 civilians living in Hudaydah could be affected by the onslaught.

The ground forces, loyal to the internationally-recognized government of Abed Rabbo Mansour Hadi, are to receive aerial support from the Saudi-led Arab air force coalition in what was the largest military operation since it intervened in the Yemeni conflict in 2015.

The attack came just days after the UAE leveled an ultimatum calling for the Houthi insurgents to surrender the city and allow for a civilian evacuation, while the Gulf nation's foreign affairs minister Anwar Gargesh claimed the "liberation" of Hudaydah would spell a "new reality" in the conflict that would boost the possibility of negotiations between warring parties.

An important quantity of food and military supplies imported into Houthi-controlled areas of Yemen pass through Hudaydah.

The Houthis, a majority Shia armed group allied to regional powerhouse Iran, seized power in the capital Sana'a and eastern regions of the country between 2014-15, forcing the Hadi government into exile, from where it requested armed support from its Sunni neighbor Saudi Arabia.

The ensuing civil war plunged the nation into a humanitarian crisis. According to the UN, which has limited access to Yemen, hundreds of thousands of people have fled fighting and as many as 8 million people were on the brink of starvation.