UN special envoy Martin Griffiths told the Security Council that there had been a dramatic deterioration in the more than six-year-long war with a Houthi offensive on Marib, putting millions of civilians at risk.
UN humanitarian agency says the risk of large-scale famine in the Arab world’s poorest country “has never been more acute."
Hudaida, Yemen's main port and a lifeline for millions of Yemenis on the brink of starvation, became the focus when the Saudi-led coalition tried twice to seize the port to cut off the Houthis' main supply line.
The Saudi-led coalition says it attacked an area in Aden that allegedly poses a "direct threat" to the Saudi-backed government of Abd Rabbu Mansour Hadi. The separatists began withdrawing shortly after.
Up to 60 percent of beneficiaries at seven centres in Sanaa "confirmed they had not received any assistance" and 33 percent of respondents in the rebels' northern stronghold of Saada received no food in April, says head of the UN food agency.
As part of the "phase one" of a ceasefire, the Houthi movement's unilateral withdrawal from the key ports began on Saturday, in the most significant advance yet for efforts to end the four-year-old war and relieve hunger.
Mohammed Ali al Houthi, the head of the rebels' Supreme Revolutionary Committee, says the pullout started on Saturday at 0700 GMT. But the Yemen government says the rebels are faking withdrawal.
As the conflict entered its fifth year on Tuesday, figures showed at least half a million children have dropped out of school with many doing odd jobs to sustain themselves and their families.
Warring parties exchanged fire on Sunday into Monday as the UN seeks to prop up a ceasefire in the key port city Hudaida, a lifeline for millions at risk of starvation after five years of bombing of Houthi rebels by a Saudi-led coalition.
Norwegian Refugee Council says sniper and explosive attacks have been growing despite an easing of air attacks amid a three-month-old truce in Hudaida, as the devastating conflict intensifies elsewhere.
Fresh fighting comes days after Yemeni government accused Houthi rebels of breaching ceasefire and refusing to withdraw from port city in line with December agreement.
Significant donations came from Saudi Arabia and the United Arab Emirates, the two main countries in a coalition supporting the government side in Yemen's conflict.
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