(Reuters) - The United Nations seeks to raise over $4 billion at a pledging event on Wednesday for war-torn Yemen where the humanitarian drive has seen funding dry up even before global attention turned to the crisis in Ukraine.

       More than 17 million people in Yemen need food assistance and this could rise to 19 million in the second half of the year, U.N. bodies said.

       "While Ukraine understandably and rightly requires our urgent attention and focus right now, we cannot drop the ball on other crises," said Swedish foreign ministry official Carl Skau.

       U.N. aid chief Martin Griffiths told the same news briefing that aid agencies were already forced to cut back or stop food, health and other vital assistance in Yemen where the economy and basic services have collapsed in the seven-year war.

       "This year's response needs nearly $4.3 billion," he said.

       Food prices, which doubled last year due to a blockade imposed by a Saudi-led coalition battling Yemen's Houthi group, are set to rise further since a third of the country's wheat comes from Russia and Ukraine.

       In Aden's Keraa camp, Abdo Yehya told Reuters they have seen no aid this year.

       "We survive with the help of our son who collects empty plastic bottles and metal cans and sells them, and...the kindness of people," he said. "We are exhausted."

       The U.N. received just over half the $3.4 billion needed in 2020 while last year, donors gave $2.3 billion.

       The World Food Programme warned on Monday that without substantial new funding mass starvation and famine would follow.

       Donor budgets were strained by the pandemic, the Afghanistan crisis and now Russia's invasion of Ukraine. There are also concerns over allegations of Houthi interference in aid flows.

       The Houthis ousted the government from the capital, Sanaa, in late 2014, prompting the coalition to intervene months later.

       (Reporting by Ghaida Ghantous in Dubai and Yemen team; Editing by Cynthia Osterman)