The situation in El Fasher, the capital of Sudan's North Darfur state, grows more dire by the day as the siege tightens, UN humanitarians said on Monday.

"Satellite images show that berms surrounding the city have now extended more than 68 kilometres, narrowing the remaining gap to as little as three to four kilometres, further constraining options for movement into or out of the area," said UN Office for the Coordination of Humanitarian Affairs (OCHA).

"The images also show large groups of people attempting to flee on foot from within the berm-enclosed area. Local sources reported that civilians trying to leave the city have faced violence, harassment and looting along insecure routes," it added.

"OCHA once again calls for the protection of civilians in El Fasher as required by international humanitarian law, as well as for safe passage for those who wish to leave," the office said.

An association of Sudanese doctors said that more than 20 people in El Fasher, including children and pregnant women, have died of malnutrition this month.

The deaths highlighted the urgent need for humanitarian access to reach the besieged population with life-saving assistance, OCHA said.

The office said the humanitarian situation is also dire in the Kordofan region, where conflict has restricted access to food, essential supplies, banking services and humanitarian aid, and has damaged critical infrastructure.

The International Organization for Migration reported that more than 1 million people have been displaced in the region since the conflict started two and a half years ago.

Seasonal flooding also continues to increase humanitarian needs in Sudan.

OCHA said that in Aj Jazirah state, local sources reported that floodwaters have destroyed farms in the Janub Al Jazirah locality, affecting 15,000 people. Residents urgently require shelter, food and essential medicines.

The humanitarian office said that stepped-up international support is urgently needed to sustain and scale up life-saving operations across Sudan.