Two rebel groups from Sudan's Darfur region say they will fight alongside the army in the country's civil war.

This comes after the paramilitary Rapid Support Forces (RSF) made major gains in Darfur, where it has been accused of ethnic cleansing.

Rebel leader Gibril Ibrahim told BBC Newsday they "want to defend their civilians" from the RSF, which he says has been burying people alive.

He said the decision to join forces with the army was not an easy one.

The leader of the Justice and Equality Movement (Jem) said it had taken seven months to come to an agreement.

The relationship between Jem and the Sudanese army is fraught. Mr Ibrahim's brother was killed by the army, who was previously the group's leader.

Jem and the Sudanese Liberation Movement (SLM) took up arms in Darfur in 2003, accusing the government of marginalising the region's black African communities.

The government then mobilised Arab militias against them, leading to what has been described as the 21st Century's first genocide.

These militias have since transformed into the RSF, which has been fighting the army for control of the country since April.

The RSF has taken several key towns in Darfur in recent weeks, including the country's second biggest city, Nyala.

Last week, there were reports they had massacred hundreds of people in the West Darfur capital of El Geneina.

The RSF has denied responsibility for the killings, saying they were part of a "tribal conflict".

A joint statement from Jem and the SLM read: "We have announced ending any neutrality and joining the military operations at all frontlines without hesitation."

Mr Ibrahim expressed concern at the RSF's advances, saying he feared Sudan would end up divided.

There are fears it could declare its own government in the impoverished war-torn western region.