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Battles raged in Sudan's war-torn capital of Khartoum on Tuesday, witnesses said, and the residents of an island in the Nile reported being "under siege" amid desperate shortages.

Eight weeks of fighting have pitted army chief Abdel Fattah al-Burhan against his former deputy Mohamed Hamdan Daglo, who commands the powerful paramilitary Rapid Support Forces (RSF).

A number of broken ceasefires have offered brief lulls but no respite for residents of the city, where witnesses again reported "the sound of heavy artillery fire" in northern Khartoum.

Witnesses also said there were "clashes with various types of weapons" in south Khartoum, where "the sound of explosions shook our walls".

In the city centre, at the confluence of the White Nile and Blue Nile rivers, the island of Tuti is "under total seige" by RSF forces, resident Mohammed Youssef told AFP.

Paramilitaries have blocked the only bridge to the island and prevented residents from going by boat to other parts of the capital.

"We can't move anyone who's sick to hospitals off the island," Youssef said.

"If this continues for days, stores will run out of food."

Since the fighting began on April 15, more than 1,800 people have been killed, according to the Armed Conflict Location and Event Data Project.

The United Nations says that more than a million and a half people have been displaced, both within the country and across its borders.

For those still in Khartoum and the western region of Darfur -- which together have seen the worst of the fighting -- the situation is growing increasingly dire.

"We face a massive humanitarian crisis that is only going to get worse with the collapse of the economy, collapse of the health care system," the International Federation of Red Cross and Red Crescent Societies (IFRC) warned on Tuesday.

The danger will increase with "the flood season fast approaching and the looming hunger crisis and disease outbreaks that now are becoming more inevitable".

Sudan's annual rainy season begins in June, and medics have repeatedly warned that it threatens to make parts of country inaccessible, raising the risks of malaria, cholera and water-borne diseases.

Some 25 million people -- more than half the population -- are now in need of aid and protection, according to the UN.

More than 425,000 people have fled to other countries -- more than 100,000 west to Chad and 170,000 north to Egypt.

"There's an urgent need for a massive injection of funds" to support those fleeing the violence, according to the IFRC.

The UN has also appealed for financing, as the fighting shows no signs of abating.

Washington slapped sanctions on the two warring generals last week, blaming both sides for the "appalling bloodshed" after a US- and Saudi-brokered truce collapsed and the army pulled out of ceasefire talks altogether.

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