Sudanese President Omar al-Bashir started a tour of Darfur on Friday ahead of a referendum on whether to keep the conflict-torn western area as five states or to create one united region.

Bashir -- who is wanted over war crimes allegations in Darfur -- said he is holding the vote under a 2011 agreement between Khartoum and some of the rebel groups that have been battling his forces for more than a decade.

The ruling National Congress Party back the five-state system but some opposition parties and Darfur insurgents have said the time is not suitable for a referendum.

"It is the people of Darfur who choose whether they want states or one region," Bashir told crowds of cheering supporters in North Darfur State capital El Fasher.

"We want all people to go to vote and participate," he said in the speech, the first on a five-day tour that will take him to every Darfur state capital before voting starts on April 11.

Darfur is home to myriad ethnic and tribal groups and it was a single region until 1994, when the government split it into North, South and West Darfur states.

Two more states, East and Central Darfur, were created in 2012.

The region erupted into conflict in 2003 when ethnic minority insurgents rebelled, complaining the region was being economically and politically marginalised by the Arab-dominated government in Khartoum.

The government mounted a counter-insurgency using regular troops, allied militia and aircraft.

The International Criminal Court indicted Bashir charges of war crimes, crimes against humanity and genocide, all of which he has denied.

Some 300,000 people have been killed in the conflict and there are 2.5 million people in the region who have been forced to flee their homes, the United Nations says.

Although many insurgents have previously called for a united Darfur region with greater autonomy, they do not want a referendum now due to current unrest.

There has been heavy fighting in Darfur's Jebel Marra mountains since mid-January, with tens of thousands reported to have fled.

But the Sudanese government is adamant the three-day referendum will go ahead.

The number of people in the western region signed up for the referendum has reached "3,583,105 out of 4,588,300 entitled to register," senior referendum commission official Adam Daleel told AFP.

The referendum was provided for under the Doha Document for Peace in Darfur signed in 2011 by the Sudanese government and the rebel Liberation and Justice Movement.