South Sudan's ruling party said Wednesday that the 60-per cent turnout threshhold required for a landmark independence vote to be valid has been reached after just three days of polling.

The former rebel Sudan People's Liberation Movement cited its own estimates for the achievement which has to be officially confirmed by the South Sudan Referendum Commission that is organising the week-long vote.

But there were long queues at polling stations again on Wednesday as voters responded to calls from the party for a "100 per cent turnout," although the shine was taken off the jubilant mood in the south by deadly tensions over the flashpoint district of Abyei on the border with the north.

"The mark that would make the referendum valid is past," said senior SPLM official Anne Itto.

"The 60-per cent threshold has been achieved but we. Are asking for a 100-percent turnout."

Referendum commission spokeswoman Suad Ibrahim said the SPLM's boast was plausible. "It's quite possible. It could even be higher," she told AFP.

Ito called on those who had not cast their ballots to do.

"Voting is an honour to those who sacrificed their lives so we can have this choice today," she said, referring to the devastating 1983-2005 civil war with the north in which an estimated two million people died.

"It is a great opportunity that will not happen again."

Loudspeaker trucks criss-crossed the potholed dirt tracks of Juba urging voters to respond to the call.

"It is important that you go out to vote, have your cards with you. You have only three days more, freedom can't wait," the hailers intoned to the accompaniment of loud African music.

The shine was taken off the party mood by the deaths of 10 southerners returning from the north for the referendum in an ambush by heavily armed Misseriya Arab tribesmen on Monday.

The attack was the latest incident in an upsurge of violence over Abyei, a disputed district on the north-south border where the nomadic Misseriya are engaged in bitter clashes with pro-southern Dinka farmers over the its future, which have claimed at least 33 lives since Friday.

Sudanese interior minister Ibrahim Mohammed Ibrahim was due to hold talks with Misseriya leaders in Abyei on Wednesday after South Kordofan provincial governor Ahmed Mohammed Harun made a controversial visit to the UN-patrolled district on Sunday.

The United Nations came in for criticism from human rights groups for providing a helicopter flight for Harun, who is wanted for the international criminal court for war crimes and crimes against humanity over the eight-year-old conflict in Sudan's western region of Darfur.

UN spokesman Martin Nesirky said Harun was "critical to bringing the Misseriya tribal leaders in southern Kordofan to a peace meeting in Abyei to stop further clashes and killings."

The head of the UN mission in Sudan, Haile Menkerios, was in district on Tuesday as its peacekeepers stepped up patrols.

UN chief Ban Ki-Moon condemned the violence while British Foreign Secretary William Hague called for restraint from the rival ethnic groups.

Abyei had been due to hold a plebiscite on its own future alongside the southern referendum, but it has been indefinitely postponed amid deadlock between northern and southern leaders over who should be eligible to take part.

The Misseriya, who migrate to Abyei each dry season to find water and pasture for their livestock, insist they should have the same right to vote as the Dinka, who live in the district all year.

Ambassador Princeton Lyman, the lead US negotiator with Sudan, said the situation in Abyei was "worrisome" but added that "nothing in these unfortunate situations is impacting on the referendum" in the south.

Lyman said that Washington was prepared to reconsider khartoum's presence on the US list of state sponsors of terrorism if the northern government acknowledged the legitimacy of the referendum result.

"Should the referendum be carried out successfully and the results are recognised by the government, President (Barack) Obama would indicate his intention to begin the process of removing them," he said.