JUBA, Sudan (AP) — Three bomb attacks in Southern Sudan this month made by aircraft from the northern Sudanese military violated Sudan's 2005 peace agreement, a joint north-south committee has concluded, a United Nations spokesman said.

A committee with representatives from the U.N. mission in Sudan and the northern and southern Sudanese militaries found that the bombings violated the agreement that ended more than 20 years of civil war, U.N. spokesman Kouider Zerrouk said Thursday.

The bombings in western Bahr el Ghazal state happened Dec. 6, Dec. 8 and Dec. 9. No casualties were reported, but they follow multiple bombing runs by the north in November in a disputed region on the border between neighboring northern Bahr el Ghazal state and southern Darfur state.

The members of the committee did not characterize the November bombings as a violation of the peace agreement, but both the northern and southern militaries agreed that those incidents were "unfortunate and should not be repeated."

National Security Council spokesman Mike Hammer said the U.S. was concerned about the attack in light of a referendum on independence scheduled for next month.

"This attack comes at a time that we are also seeing increased evidence of support to militant proxies from the Governments of Sudan and Southern Sudan. All Sudanese leaders have a responsibility to protect civilian populations — to do otherwise is unacceptable."

Southern Sudan is scheduled to hold a Jan. 9 independence referendum that is likely to see Sudan — Africa's largest country — split in two. The 2005 peace accord gives the south the right to the vote. Diplomats, the U.N. and the African Union are working behind the scenes to ensure a peaceful referendum.

An Associated Press reporter last week visited the sites of the November bombings and saw multiple bomb craters and evidence that straw huts burned to the ground after the explosions. Southern military officials said they believed the bombings were provocations intended to ignite conflict and disrupt the referendum.

Thousands of civilians fled the site of the November bombings, and the southern military brought in three anti-aircraft guns as a defensive measure. The total wounded in the attacks is believed to range from 16 to 22, with 10 to 11 of those being soldiers. No deaths were reported.

The joint U.N.-Sudanese committee — known as the Ceasefire Joint Military Committee — recorded the December bombings as a violation of the 2005 agreement despite an objection from the senior member from north Sudan, the U.N. said.

Meanwhile, the U.N.'s human rights chief told The Associated Press on Thursday that Sudan's government is blocking aid workers from entering the country ahead of next month's referendum. Navi Pillay, the U.N. High Commissioner for Human Rights, said human rights observers and aid workers need to be in place before Jan. 9 to prepare for possible unrest.

"There's a huge holdup of visas, almost 1,000," Pillay told the AP. "It just seems deliberate, this holdup on visas. I hope the Secretary-General (Ban Ki-moon) will address this directly with the government."

"It will really hamper human rights and humanitarian work if we're not there on the ground in time," she said.

An official who answered the phone at Sudan's mission in Geneva described claims of a delay in issuing visas as "baseless." The woman, who gave her name as Naima Lazaar, said senior embassy officials were unavailable to discuss Pillay's claim.

The Carter Center — the largest international mission observing the referendum — said late Wednesday that the three-week registration process of southern voters was "generally credible." The final voter list is not scheduled to be published until Jan. 8, one day before voting begins.

A group of lawyers from Khartoum has said it will file a lawsuit to challenge the registration process, and a group claiming to represent Southern Sudanese groups filed a similar suit.

The head of the referendum commission's bureau in the south has dismissed the complaints as "baseless, frivolous, and politically motivated."

"If the referendum is delayed, I think the repercussions could be great in terms of violence and in terms of confidence and the trust between north and south," said Chan Reec Madut, the head of the Southern Sudan Referendum Bureau.

The Southern Sudan Referendum Commission says more than 3.2 million people registered to vote in the south — 96 percent of the eligible population.

Northern and southern officials continue to negotiate over key issues such as citizenship, wealth-sharing, demarcation of the border and the status of the oil-rich region of Abyei. That region had also been scheduled to decide on Jan. 9 whether to join the north or south, but the U.S. said last week that Abyei's vote won't happen and that the region's status is likely to be decided through talks.

The U.N. Security Council on Thursday urged the north and south to ensure a peaceful referendum on Jan. 9 and agree on a separate referendum to resolve the fate of Abyei.

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Associated Press writers Frank Jordans in Geneva and Edith Lederer in New York contributed to this report.