JUBA, Sudan: South Sudan was a step closer to becoming the world's newest state on Monday after vast crowds of jubilant voters turned out on the first day of a landmark independence referendum.

Thousands of voters had begun waiting from hours before dawn on Sunday, eager to be among the first to have their say on whether the impoverished south should break away from rule by Khartoum, putting the seal on five decades of north-south conflict.

When the polls finally opened at 8:00 am, the excitement was electric. Each time another vote was inserted in the ballot box, women began ululating in celebration.

Southern leaders had urged voters to turn out en masse on the first day. A 2005 peace deal requires a turnout of at least 60 per cent for the referendum to be valid. The outcome will then be decided by simple majority.

Similar crowds on Monday's second day of voting in the week-long referendum will set south Sudan well on the way to achieving that turnout threshold if the scenes in the region's towns are replicated in the countryside.

A few voters wore their opinions on their chests with T-shirts demanding independence.

Most donned the Western suits and brightly coloured floral dresses normally reserved for church in this largely Christian region, but they showed no less enthusiasm for separation from the Muslim, mainly Arab north.

US President Barack Obama, who aides said had voiced a deep personal commitment to ending the north-south conflict in Sudan, hailed the successful first day of voting and pledged continued support from Washington.

“I am extremely pleased that polling has started for the southern Sudan referendum, and congratulate the people of southern Sudan who are determining their own destiny,” Obama said.

“The United States will remain fully committed to helping the parties solve critical post-referendum issues regardless of the outcome of the vote,” he added.

While the first day's voting was mostly peaceful, an official said that clashes between renegade militiamen and south Sudanese troops disrupted voting in part of the key oil Unity state, which abuts the north-south border.

“There has been some fighting because of certain militiamen but I am assured that the situation has been contained,” said Chan Reec, the organising commission's number two.

Reec, who had earlier been effusive about the massive turnout, later appealed for more consideration to be shown to women and the elderly at polling stations.

Polls had been scheduled to close at 5:00 pm, but many polling stations visited by AFP in the regional capital Juba stayed open for another two hours or more to deal with the huge backlog of eager voters.

Some 3.75 million people are registered to vote in the south and around 117,000 in north Sudan, the majority in the capital Khartoum. Émigrés were also able to vote in eight countries abroad.

Final results are not expected until next month because of the problems involved in collecting ballot boxes in a vast, war-ravaged region which has just 40 kilometres of tarmac road.

The independence referendum is the centrepiece of the 2005 north-south peace deal that ended a devastating 22-year civil war in which some two million people were killed and another four million fled their homes.

South Sudanese president Salva Kiir was among the first to cast his ballot in Juba.

“This is the historic moment the people of south Sudan have been waiting for,” Kiir said, holding up his hand to reporters to show the indelible ink that demonstrated he had voted.

US envoys Scott Gration and John Kerry as well as Hollywood star George Clooney watched as Kiir cast his ballot.

Sudan President Omar al-Bashir, an army man who led the north's war against the south for a decade and a half before signing the peace deal six years ago, has said he will respect the vote's outcome if it is “free and transparent.”