Clashes reignited between the Sudanese army and rival paramilitaries earlier this week in the key Darfur town of El-Fasher, the United Nations said on Sunday, killing at least 27 people in one day.

Eyewitnesses have reported air strikes, artillery fire and machine gun clashes battering the city since Friday, when an hours-long battle left an estimated 850 people displaced, according to the UN.

It also killed at least 27 that day, based on what the UN said were "unconfirmed reports", as the city suffers a near-total communications blackout, with medics and human rights defenders barely able to get news to the world.

The fighting has since continued, eyewitnesses said Sunday, reporting air strikes and artillery shelling that left "houses on fire", one resident told AFP.

According to French medical charity Doctors Without Borders (MSF), two children and a caregiver were killed in an intensive care unit Saturday following a nearby military air strike.

Since April of last year, Sudan has been in the grips of a devastating war between the army, headed by the country's de facto leader Abdel Fattah al-Burhan, and the paramilitary Rapid Support Forces (RSF), commanded by his former deputy Mohamed Hamdan Daglo.

The RSF has seized four out of five state capitals in Darfur, a region about the size of France and home to around one quarter of Sudan's 48 million people.

El-Fasher is the last major city in Darfur that is not under paramilitary control. The international community, including the UN and the United States, have for weeks warned against a looming offensive on the city.

MSF said Sunday that an air strike carried out by the army -- which maintains a functional monopoly on the skies -- landed 50 metres from the Babiker Nahar Paediatric Hospital.

It caused the roof of the ICU to collapse, resulting in "the death of two children who remained receiving treatment there, as well as the death of at least one caregiver," according to a statement.

"The children who were killed were in a critical condition in our ICU, but their lives could have been saved," MSF's head of emergency operations Michel-Olivier Lacharite said. (AFP)