JUBA, Sudan -- Arab tribesmen accompanied by a northern Sudanese government-backed militia killed 20 policemen in the disputed region of Abyei, a southern military spokesman said Monday, raising concerns of violence as the south holds its independence referendum.
The attack came Sunday, the first day of voting in Southern Sudan's weeklong referendum, which is widely predicted to break Africa's largest country in two.
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 Khartoum-backed militia whose existence was outlawed by the 2005 peace agreement that ended the 23-year north-south civil war.
Aguer said 20 police serving with Abyei's joint integrated police unit were killed. Another 30 were wounded.
Jubilant voters flooded polling stations in Southern Sudan on Sunday and again on Monday, and the seven days of balloting are likely to produce an overwhelming vote for independence. Sudan President Omar al-Bashir has said he will let the 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 reverse the south's gains and send the north and south back toward conflict. Clooney helped set up a project for satellites to watch any possible troop movements in Abyei.
Carter told The Associated Press on Sunday that Abyei is a flashpoint that concerns him.