A UN official on Tuesday warned armed factions in Yemen over "the deliberate starvation of civilians", as the embattled country grapples with a food crisis that has left 850,000 children facing acute malnutrition.
Six million people are "severely food insecure", the UN's special rapporteur on the right to food, Hilal Elver, said in a statement.
Armed groups besieging population centres were partly to blame, Elver said.
"Sieges in a number of governorates, including Aden, AL Dhali, Lahj and Taiz, have been preventing staple food items, such as wheat, from reaching the civilian population," the statement said.
She noted that air strikes have reportedly targeted both markets and trucks carrying food.
"The deliberate starvation of civilians in both international and internal armed conflict may constitute a war crime," she further said, in a warning that did not assign blame to any specific group.
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Saudi-backed pro-government forces have been battling for months to restore President Abedrabbo Mansour Hadi, in exile in Riyadh, to power.
They have made major gains in recent weeks over Iranian-backed rebels, who, despite recent losses, have vowed to reclaim all lost territory.
The food crisis is set to deteriorate further, according to Elver, with the number of children facing acute malnutrition in Yemen due to rise to 1.2 million "in the coming weeks".
The World Health Organization on Tuesday said the conflict had claimed 4,345 lives from the escalation of fighting in March to August 5.
After a three-day visit to the country, the head of the International Committee of the Red Cross (ICRC), Peter Maurer, said "Yemen is crumbling."
"The humanitarian situation is nothing short of catastrophic," he added in a statement.
He echoed the UN's call for the unrestricted movement of food, water and medicine, while highlighting the collapsing the health services.
The WHO said health professionals are fleeing Yemen to escape the violence and that the health care system "continues to shut down."