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rebels have taken control of a large area south of , a lifeline port where the warring sides agreed a ceasefire in 2018, government military sources said Friday.

The rebels advanced after to the internationally recognised government retreated from positions south of the Red Sea port city, the sources said, without giving reasons.

The Hodeida ceasefire was agreed at Yemen's last peace talks in Sweden in 2018, but clashes have since broken out between the rebels and pro-government troops around the city.

Elsewhere, the -backed insurgents, who are fighting a Saudi-led coalition, have been engaged in a sustained attempt to capture Marib, the government's last stronghold in the north.

The United Nations said it was aware of the reported withdrawal near Hodeida and the Houthis' move into the vacated areas, and was watching the situation closely.

"We call on all parties to the conflict to ensure the safety and security of civilians in and around those areas where shifts in frontlines have taken place," the UN secretary general's deputy spokesman, Farhan Haq, told a regular briefing.

The coalition intervened militarily to prop up the Yemeni government in 2015, after the Houthis seized the capital Sanaa the year before.

Since the start of the conflict, tens of thousands of people have been killed and millions displaced, precipitating what the UN has described as the "world's worst humanitarian crisis".