Estimates that about a million people might flee Sudan by October may be conservative and conflict there risks increasing people trafficking and spreading weapons across a fragile region, the head of the U.N. refugee agency said on Monday.

More than 350,000 people have already fled across Sudan's borders since war between the army and the paramilitary Rapid Support Forces (RSF) erupted on April 15, with most heading to Egypt, Chad and South Sudan.

More than 1 million have been displaced within Sudan, which has a population of 49 million and where heavy fighting has torn through residential areas of the capital Khartoum and violence has also flared in the western region of Darfur.

UNHCR had foreseen about 800,000 Sudanese and 200,000 people of other nationalities leaving Sudan over six months, the refugee agency's head Filippo Grandi said in an interview in Cairo after a visit to the border with Sudan.

"This projection, that in the next few months we'll reach these high figures, may even be conservative," he said. "At the beginning I didn't believe it would be, but now I'm beginning to be worried."

The nations bordering Sudan include South Sudan, Central African Republic, Ethiopia, and Libya, all affected by their own recent conflicts.

The collapse of law and order in Sudan and "a lot of people desperate to move on" would provide fertile ground for human trafficking, while arms circulating across borders could engender more violence, said Grandi.

"We've seen it in Libya with the Sahel. We don't want a repeat of that because that will be a multiplier of crisis and of humanitarian problems," he said.

The United Nations has appealed for $470 million for its refugee response to the Sudan crisis over six months, an amount that Grandi said was just 1% funded, adding that a donor pledging conference was "very much needed" and that an international community preoccupied by Ukraine was not paying enough attention.

"You can clearly sense a disparity which is very dangerous. This crisis has the potential to destabilise an entire region and beyond as much as Ukraine does in Europe," he said.

Grandi said UNHCR was trying to establish a presence in the northern Sudanese town of Wadi Halfa, where many Sudanese men aged 16-50 have become stuck applying for visas to enter Egypt, but that he was not sure when this would be possible. Women, children and the elderly do not need visas.

He also said aid needed to be delivered into a buffer zone between the Egyptian and Sudanese border posts where those fleeing have also faced long waits.

Since the conflict began, nearly 160,000 people have crossed from Sudan into Egypt, which was already home to a large Sudanese community.

Smoke billows in Sudan's capital Khartoum on the fifth day of a US-Saudi brokered ceasefire
AFP