Hundreds of thousands of civilian lives "hang in the balance" in Yemen's Hodeida province, where families are living in fear of shelling and air strikes, the UN said on Thursday.
"The situation has dramatically deteriorated in the past few days... people are struggling to survive", said the UN's humanitarian coordinator for Yemen, Lise Grande, in a statement.
Doctors and medics in two hospitals in Hodeida province said 50 people have been killed in the past 24 hours, seven of them pro-government fighters.
The UN's attempts to broker peace talks between Houthi rebels and Yemen's Saudi-backed government collapsed on Saturday.
Fighting has raged in the last two days close to the rebel-held port city of Hodeida, a crucial entry point for humanitarian aid that the Saudi-led coalition alleges also serves as a key conduit for arms to the Iran-backed Houthis.
Alongside the threat of combat, civilians also face severe shortages of food, water and medicine in Hodeida province, Grande said.
More than a quarter of children are malnourished, 900,000 people are desperate for food and 90,000 pregnant women "are at enormous risk," she said.
The situation would be exacerbated if the fighting compromises mills and stores in Hodeida province.
"We're particularly worried about the Red Sea mill, which currently has 45,000 metric tonnes of food inside, enough to feed 3.5 million people for a month," Grande said.
"If the mills are damaged or disrupted, the human cost will be incalculable."
Coalition forces on Wednesday seized rebel supply routes into Hodeida city. Clashes were still ongoing on Thursday, military sources said.
The Houthis launched a counter attack to retake Kilo 16 -- the main supply route -- while there was also fighting over the other road, Kilo 10, the same sources said.
Government forces backed by the coalition had before this week paused their assault on Hodeida in what they said was a bid to give the UN-led peace efforts a chance.
But the rebels refused to send a delegation to Geneva for talks, complaining the UN had been unable to guarantee safe passage back to Sanaa or an evacuation of wounded fighters to nearby Oman.
Nearly 10,000 people have been killed in Yemen's conflict since 2015, when the Saudi-led alliance intervened in support of the government.