* Sudan says vote will settle dispute at heart of conflict

* Rebels say vote will be rigged, students protest

* Speculation grows that Darfur could follow south to independence (Adds rebel quote, recent fighting, background on ICC)

By Khalid Abdelaziz

El FASHER, Sudan, April 11 (Reuters) - Darfur residents began voting on Monday in a referendum on whether to reunite their arid western region into one entity, a poll Sudan says will settle an issue at the heart of the 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 the large Fur tribe said the break-up allowed Khartoum to dive and rule them.

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

University students in El Fasher, the government-controlled capital of North Darfur state, protested against the vote and witnesses said similar rallies took place in at least three refugee camps in Central Darfur state.

Turnout was strong in the centre of El Fasher, where security forces were out in force, but weak outside the city.

"I will not take part in the referendum as the results are already known. The option of states will win as this is what the government wants. This referendum is meaningless," said one man at Abu Shouk refugee camp outside El Fasher.

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

But there were also voters who wanted to retain the states. "We came out since morning to give our opinion... I want the states system, it's best for us," Samia Abkar, a 24-year-old woman in tattered clothing, told Reuters at a polling centre.

REBEL BOYCOTT

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

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

Although violence has eased in recent years, the insurgency continues and Khartoum has escalated attacks on rebels over the past year. At least 130,000 people have fled fighting in the central Jebel Marra area since mid-January alone.

The two main Darfur rebel groups, the Justice and Equality Movement (JEM) and the Sudan Liberation Army, have urged their followers to boycott the three-day referendum.

"We reject this referendum and will not concede its outcome because the referendum should be held after we reach a comprehensive political settlement ... and the displaced and refugees return to their villages," said Jibril Belal, spokesman for JEM.

The administration of Darfur is a sensitive issue for President Omar Hassan al-Bashir, who saw a 2005 peace deal with rebels in the south lead to the reunification of that region and an eventual referendum on independence, which Khartoum lost.

South Sudan, roughly the same size as Darfur, became independent in 2011. Darfur, to the west of Sudan, was once independent, becoming a part of Sudan in 1916 and being ruled as a single province for decades after.

Bashir, wanted for genocide by the International Criminal Court (ICC) over his campaign to crush the Darfur rebellion, toured the region over several days last week, saying the situation was stable and encouraging people to vote. (Writing by Lin Noueihed and Amina Ismail; Editing by Tom Heneghan)