Millions of Ukrainians may soon face serious food insecurity because of the ongoing conflict amid reports that people are facing "life-and-death decisions" on whether to leave Mariupol and elsewhere, the UN said Tuesday.

More than a month since the Russian invasion, the alert from the Food and Agriculture Organization (FAO) follows assessments in 19 of Ukraine's 24 regions, which indicated that it was uncertain if the country could harvest crops, plant new ones or sustain livestock production.

"An immediate and worrying finding is that food shortages are expected immediately or in the next three months in over 40 percent of the surveyed areas and cases," said Rein Paulsen, FAO director of the Office of Emergencies and Resilience.

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"When it comes to the all-important production of vegetables, conflict is likely to severely disrupt production for tens of thousands of smallholder farmers, those who have decided to stay behind."

In Mariupol and other Ukrainian locations that have been encircled and pummelled by indiscriminate shelling, the International Committee of the Red Cross (ICRC) warned that the crisis was "deepening."

According to the UN aid coordinating office, OCHA, around 90 percent of Mariupol's residential buildings – some 2,600 homes – have been affected by active fighting.

Some 60 percent of buildings have suffered various degrees of damage from direct shelling, while about 40 percent have been completely destroyed.

Local authorities say the rising civilian death toll is fast approaching 5,000 people.

Highlighting the risks faced by medical workers and patients in Ukraine, Dr Jarno Habicht, World Health Organization representative in the country, said there have now been "74 attacks, with 72 deaths and 40 injuries between February 24 to March 25…These attacks are against hospitals, ambulances."