The UN on Monday more than doubled its aid appeal to assist nearly two million people expected to flee conflict-torn Sudan by the end of this year amid soaring needs.

The United Nations refugee agency (UNHCR) said it and partners were appealing for $1 billion "to provide essential aid and protection to over 1.8 million people expected to arrive in five neighbouring countries by the end of 2023, fleeing ongoing conflict in Sudan".

"This is a two-fold increase of what was initially estimated in May to be required to respond to the crisis, as displacement and needs continue to soar," UNHCR said in a statement.

The agency had initially appealed for $445 million for the regional refugee response this year, and already hiked the estimate twice prior to Monday's announcement.

Since war began between the regular army and the paramilitary Rapid Support Forces on April 15, around 5,000 people have been killed, according to conservative estimates from the Armed Conflict Location & Event Data project.

The violence has spurred a massive exodus.

Less than five months into the crisis, more than one million refugees, returnees and third country nationals had fled Sudan, often into dire situations in neighbouring countries.

"Those arriving in remote border areas find themselves in desperate circumstances due to inadequate services, poor infrastructure and limited access," the UNHCR regional refugee coordinator for the Sudan situation Mamadou Dian Balde said in the statement.

"Partners active in this response are making every effort to support those who are arriving and their hosts, but without enough donor resources, these efforts will be severely curtailed."

UNHCR said that so far, only 19 percent of the $1 billion appealed for had been received, as humanitarians scramble to provide refugees critical necessities like water, food and shelter.

The dire health situation among new arrivals required particularly urgent attention, it said, pointing to high malnutrition rates and outbreaks of diseases like cholera and measles.

"It is deeply distressing to receive reports of children dying from diseases that are entirely preventable, should partners have had sufficient resources," Balde said.

UNHCR said countries receiving refugees –- Central African Republic, Chad, Egypt, Ethiopia and South Sudan –- were already hosting hundreds of thousands of displaced people before this crisis.

"Countries in the region are facing major challenges of their own," Balde said, insisting that "we cannot take their hospitality for granted".

"The international community needs to stand in solidarity with host governments and communities and address the persistent underfunding of humanitarian operations."