JUBA, Sudan -- Arab tribesmen accompanied by a Khartoum-backed militia killed 20 policemen in Sudan's disputed region of Abyei, a southern military spokesman said Monday, raising concerns of violence as the south carries out a weeklong independence referendum.
A tribal leader, though, said that police had killed 10 herders in the area. The reported attacks came Sunday, the first day of Southern Sudan's self-determination vote, which is widely predicted to break Africa's largest country in two. The referendum was promised in a 2005 peace deal that ended two decades of civil war.
Jubilant voters flooded polling stations for a second day on Monday. The seven days of balloting are likely to produce an overwhelming vote for independence, and Sudanese President Omar al-Bashir has said he will let the oil-rich south secede peacefully.
But Abyei is still a major sticking point, and officials from former U.S. President Jimmy Carter to Sudan activist and actor George Clooney have warned that Abyei holds the potential to send the north and south back to conflict.
Abyei, which straddles the north-south divide and holds oil deposits, had been promised its own self-determination vote, but now its future will be decided by north-south negotiations that have so far made little progress.
Col. Philip Aguer, the spokesman for Southern Sudan's army, said that the Misseriya -- an Arab tribe that moves its cattle herds through Abyei -- attacked the village of Maker-Adhar on Sunday with anti-tank weapons and artillery. Aguer said he believes the attack was planned.
"They were not with cattle, they were coming for (an) attack," Aguer said.
Aguer said the Misseriya were accompanied by uniformed militia men known as the Popular Defense Forces, a militia backed by the Sudanese government in Khartoum whose existence was outlawed by the 2005 peace agreement that ended the 1983-2005 north-south civil war.
There was no immediate comment from the Khartoum-based government on the allegations.
Aguer said 20 police serving with Abyei's joint integrated police unit were killed. Another 30 were wounded. A U.N. official said the southern government has asked for help in evacuating the wounded police. The official was not allowed to be identified because the information hadn't been made public.
Clashes in disputed regions often produce widely differing accounts of the events. Bashtal Mohammed Salem, a Misseriya leader, told the AP that 10 Misseriya herders were killed Sunday in attacks by police in an area about 10 miles (30 kilometers) north of Abyei. Maker-Adhar, where Aguer reported the police deaths, is in the same general area.
"They want to keep us out of the area and declare independence unilaterally," he said. Salem said leaders of the Dinka, a southern tribe, and Misseriya agreed no more attacks would happen.
Meetings on Wednesday are to include the interior ministers of the south and north to regulate the presence of police in the area. Salem said southern security forces have increased their presence in Abyei, violating an agreement governing the issue.
Abyei also saw violence on Friday and Saturday, though officials from the north and south gave conflicting accounts of the casualties and the locations of the fighting.
President Barack Obama on Sunday singled out Abyei in a statement and said attacks there should cease.
Barrie Walkley, the top U.S. official in Juba, said that the governor of neighboring Southern Kordofan state in northern Sudan traveled to Abyei on Sunday to meet with the top official in the area. They signed an agreement pledging to address the conflicts between the two sides, Walkley said Monday.
The agreement "represents an important step to try to keep Abyei calm and to make sure that these small clashes don't escalate," Walkley said.
The south's army suspects that the governor of Southern Kordofan state, Ahmed Haroun, is arming militias in the area. Haroun is wanted for war crimes and crimes against humanity by the International Criminal Court for his role in the Darfur conflict in western Sudan.
Aguer, the southern military spokesman, accused Haroun of "doing the same thing he did in Darfur. He's the master minder of the whole situation."
The Sudanese president's regime is accused of unleashing Arab militias known as janjaweed, against rebels in the Western Darfur region which have committed atrocities against ethnic African towns and villages. The U.N. says some 300,000 people have died since 2003. The government denies backing the janjaweed and says the death figures are inflated.
Southerners, who mainly define themselves as African, have long resented their underdevelopment, accusing the northern Arab-dominated government of taking their oil revenues without investing in the south.
Southern Sudan is among the world's poorest regions. The entire France-sized region has only 30 miles (50 kilometers) of paved roads. Because only 15 percent of southern Sudan's 8.7 million people can read, the ballot choices were as simple as could be: a drawing of a single hand marked "separation" and another of clasped hands marked "unity."
Independence won't be finalized until July, and many issues are yet to be worked out. They include north-south oil rights, water rights to the White Nile, border demarcation and the status of the contested region of Abyei, a north-south border region where the biggest threat of a return to conflict exists.
Most of Sudan's oil is in the south, while the pipelines to the sea run through the north, tying the two regions together economically.
Associated Press writer Sarah El Deeb in Khartoum, Sudan, contributed to this report.