Residents of Sudan’s Darfur region began voting Monday on whether to reunite the arid western area’s states into one entity, a referendum that Sudan says will settle a contentious issue at the heart of a long-running conflict.

The Sudanese government’s decision to split Darfur into three states in 1994 helped fuel discontent that erupted into fighting in 2003 — rebels and many from the large Fur tribe said the breakup allowed Khartoum to divide and rule them.

Sudan, which later split Darfur further into five states, has presented this week’s referendum as a major concession. But rebel and opposition groups have cried foul, saying the vote will be rigged and calling on supporters to boycott it.

Students inside El-Fasher University, in the government-controlled capital of North Darfur state, protested the vote, and witnesses said similar rallies were held in at least three refugee camps in Central Darfur state.

Analysts and diplomats say the government opposes a unified Darfur, concerned that it would give the rebels a platform to push for independence — just as the south successfully did in 2011, taking with it most of Sudan’s oil reserves.

Turnout was strong in the center of El-Fasher, where there was a heavy presence of security forces.

The Darfur conflict began in 2003 when mainly non-Arab tribes took up arms against the Arab-led government based in the Sudanese capital, Khartoum, accusing it of discrimination and of marginalizing the area.

According to the United Nations, as many as 300,000 people have been killed in Darfur, about 4.4 million people need aid and more than 2.5 million have been displaced.

Although the killings have eased in recent years, the insurgency continues and Khartoum has sharply escalated attacks on rebel groups in the past year.

The two main rebel groups fighting in Darfur, the Justice and Equality Movement and the Sudan Liberation Army, have called on their followers not to take part in the three-day referendum.

They had called for a political settlement first and have warned that the referendum will lead to more violence.

South Sudan, roughly the same size as a unified Darfur, fought the north through more than two decades of civil war until a 2005 peace deal gave it the right to a referendum on whether to secede.

In 2011, southerners overwhelmingly voted to declare independence, and South Sudan became independent that year, though both countries remain at loggerheads over territorial and other issues, and the South has struggled with a civil war.