"A convoy of returnees coming from the north to the south were ambushed Monday at about 17:00 (14:00 GMT) by armed Misseriya. Ten were killed and 18 were wounded," Chuang told a news conference in the southern regional capital Juba.

Despite the killings, the landmark independence referendum has shown a big turnout on its third day, prompting tens of thousands of southerners to return from the north.

Chuang called for the Khartoum government to be held accountable for the attack by the Arab nomad tribe, which was a key auxiliary militia of the northern army during the 1983-2005 civil war and is involved in a continuing conflict with pro-southern Dinka in the disputed border district of Abyei.

Washington said on Tuesday that Sudan could be removed from the US list of state sponsors of terrorism by July if it recognized the referendum's outcome.

Upsurge of violence

"The Misseriya belong to a state and that state has to be held accountable," he said.

Misseriya chief Hamid al-Ansari denied the tribe had been involved in any ambush of returning southerners, but northern police confirmed they had received reports of an attack.

"How could we have carried out such actions when the United Nations is on the ground between us and the Dinka?" Ansari told AFP.

Sudanese police spokesperson Ahmed Tahami said: "We have received reports that a convoy of people returning to Bahr al-Ghazal (in the south) was attacked but we have no other details."

Misseriya tribesmen have stopped southerners returning to the south through their areas several times in the past as part of their conflict with the Ngok Dinka over Abyei.

There has been an upsurge of violence in the district in recent days in which the two sides reported losses totalling at least 33 dead since Friday.

Sponsors of terrorism

UN chief Ban Ki-moon on Tuesday condemned the latest violence, and the UN Mission in Sudan (UNMIS) said it had intensified its patrolling activities in Abyei and was on standby to reinforce its peacekeeping presence if needed.

The head of UNMIS, Haile Menkerios, was in Abyei on Tuesday for consultations with local leaders, a UN spokesperson said.

Western governments continued to voice their concern over the situation. A statement from British Foreign Secretary William Hague called on leaders there to exercise restraint.

In Washington meanwhile, Princeton Lyman, the lead US negotiator with Sudan, focused on what Sudan stood to gain if it respected the result of the referendum.

The United States was prepared to reconsider Khartoum's presence on the US list of state sponsors of terrorism if Khartoum acknowledged the legitimacy of the result, Lyman told AFP.

Prospect of secession

"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.

"It is a process that takes some time, but by beginning the process in the wake of the referendum, the hope is if they meet all the conditions it can be done by July," Lyman added.

In the south, referendum organisers said Sunday's and Monday's huge turnout had been repeated across the region, and that polling hours were being extended by an hour for the remaining five days of voting.

Huge crowds still queueing to cast their ballots at the end of the original 08:00 to 17:00 voting hours had left many polling stations struggling to cope over the first two days.

The referendum commission's number two Chan Reec, citing figures from less than half of polling stations, said nearly a million of the 3.75 million people registered in the south had already voted.

The prospect of secession by the south had sparked fears of a wider break-up of Sudan, which has experienced other rebellions in the war-torn western region of Darfur and also in the east, where a 12-year uprising ended with a still-fragile peace agreement in 2006.