Millions of people displaced by the war in Sudan are at risk of falling deeper into crisis as funding for food aid dwindles, the UN's World Food Programme warned Monday.

Since April 2023, war between the Sudanese army and paramilitary Rapid Support Forces has created the world's largest displacement crisis, with more than 10 million people displaced inside the country.

Another four million have fled across borders, mainly to Chad, Egypt and South Sudan.

"This is a full-blown regional crisis that's playing out in countries that already have extreme levels of food insecurity and high levels of conflict," said Shaun Hughes, WFP's emergency coordinator for the Sudan regional crisis.

The United Nations says its humanitarian response plan for Sudan -- also the world's largest hunger crisis -- is only 14.4 percent funded.

A UN conference in Spain this week aims to rally international donors, following deep funding shortfalls that have affected relief operations globally.

The WFP warned support to Sudanese refugees in Egypt, Ethiopia, Libya and the Central African Republic "may grind to a halt in the coming months as resources run dry".

In Egypt, which hosts around 1.5 million people who fled Sudan, food aid for 85,000 refugees -- 36 percent of those previously supported -- had already been cut.

Without new funding, the WFP warned, all assistance to the most vulnerable refugees would be suspended by August.

In Chad, where more than 850,000 people have fled but find little help in overwhelmed camps, the WFP said food rations would be reduced even further.

Around 1,000 refugees continue to arrive in Chad each day from Sudan's western Darfur region, where famine has already been declared and displacement camps regularly come under attack.

"Refugees from Sudan are fleeing for their lives and yet are being met with more hunger, despair, and limited resources on the other side of the border," said Hughes.

"Food assistance is a lifeline for vulnerable refugee families with nowhere else to turn."

Inside Sudan, more than eight million people are estimated to be on the brink of famine, with nearly 25 million suffering dire food insecurity.