The humanitarian crisis sparked by the civil war in Sudan is growing as neighbouring countries struggle to support refugees fleeing the violence.

The United Nations says thousands of Sudanese are crossing borders into countries already in the grip of drought, food insecurity and conflict.

The UN is in a race against time to get food supplies to Sudanese refugees crossing the border into Chad before the rainy season begins, as neighbouring countries struggle to cope with the numbers of people fleeing the civil war.

The International Organization for Migration (IOM) has launched a response plan appealing for $US209 million to provide vital humanitarian assistance to the people affected by the crisis and outbreak of violence in Sudan.

More than 120,000 people are now said to have crossed into other countries as inconsistent ceasefires mean deadly clashes are continuing between Sudanese army troops and a paramilitary rival that have killed hundreds and forced more than 330,000 from their homes.

Aid agencies have warned of hunger and disease among Sudanese crossing into neighbouring countries.

In Chad, which has seen more than 30,000 arrivals since the fighting began in mid-April, the UN’s emergency food assistance program is planning for as many as 100,000 new refugees to arrive over the coming weeks and months.

But the looming wet season threatens to cut off remote border regions.

Chad was already harbouring around 580,000 refugees from various countries before the Sudan crisis.

South Sudan has also seen almost 30,000 people arrive in recent weeks, most of them returning to a country they fled during a brutal civil war.

The Central African Republic has received about 6,000 refugees but is the world’s fourth least developed nation with few resources to support the arrivals.

Ethiopia has seen than 10,000 people arrive from Sudan> Most have crossed into Amhara which, until recently was the centre of a war between government forces and Tigrayan rebels.

The World Food Program recently cut its rations for refugees there by 40 per cent.

The UN refugee agency, UNHCR, warned this week that humanitarian operations in Sudan’s neighbouring countries were already “hugely underfunded” and needed the international community to raise money and “help us get aid into those areas where it’s desperately needed”.

The United Nations says the conflict is having devastating consequences for millions of civilians. Around 25.7 million people are now estimated to be in need of humanitarian assistance inside Sudan and across its borders.

More than 843,130 people are internally displaced inside the country according to IOM’s Displacement Tracking Matrix, with 1.8 million more expected to be internally displaced should the fighting continue. Prior to the conflict, there were already 3.8 million internally displaced persons (IDPs) and 1.1 million refugees in Sudan.

Nearly 259,000 people have already fled Sudan into neighbouring countries, arriving in extremely vulnerable situations into often remote and underserved areas. If fighting continues, it is estimated that over a million refugees and migrants affected by the crisis in Sudan may cross to neighbouring countries.

Millions remain inside the country with soaring food and transportation prices, lack of cash and access to health care, supplies and critical services.

The deteriorating situation has been marked by increasing civilian deaths, destruction of health, water and communication facilities, shortage of supplies and poor conditions at overwhelmed border points as people flee the fighting.