The New York-based organization Human Rights Watch has warned that the Sudanese government may be using militia groups on the border between northern and southern Sudan to disrupt or influence the national census scheduled to take place later this month. Derek Kilner reports from VOA's East Africa bureau in Nairobi.

Human Rights Watch noted a rise in attacks in recent months on displaced people attempting to return to their homes in Unity State and Abyei, a disputed oil-rich area between north and south Sudan.

Human Rights Watch and other observers say such attacks could be part of an effort by the Sudanese government to influence the outcome of a national census - set to take place from April 14-30.

"It looks as if this is an organized campaign," said Tom Porteous, the London Director of Human Rights Watch. "The government would say this is just normal banditry and so forth. But some of the victims we spoke to in Unity State said that it was clear that the motive of the attackers was not robbery because they simply opened fire on the travelers trying to get back to their homes without any apparent interest in stealing things from them."

The census is seen as a necessary first step before national elections in 2009 and a referendum in the south in 2011 on whether to secede from the North. There is a fear that if southerners are prevented from taking part in the census, voter registration in the south will be hampered. The population of the north and south will also determine the sharing of resources between the regions.

In 2011, residents of Abyei are also set to vote in a referendum on whether the area will be part of the northern or southern Sudan, along with its large oil reserves.

The Abyei area is home to the Ngok Dinka tribe, which has generally supported the Sudan People's Liberation Movement, former rebels who are now governing the semi-autonomous region of Southern Sudan.

The attacks have largely been carried out by militia members from the Misseriya, an Arab tribe that has traditionally allied with Khartoum, and which was armed by the Sudanese government during the North-South civil war. The Misseriya are a nomadic group that grazes cattle in the Abyei area, and the Sudanese government has attempted to have the group count as residents of Abyei.

Southern officials have accused the government of mobilizing militias in Abyei, and recently said the north has sent its own troops into Abyei town.

Rabie Abdul Atti, an advisor to Sudan's information minister, denied the northern government has sent troops to Abyei town or that it has armed members of the Misseriya.

"You know these accusations by SPLA regarding that the government sends troops to Abyei," Atti said. "This is not correct. Because the government is working very hard to solve the Abyei problem. The government does not help Misseriya, and also we did not send any troops to support Misseriya against SPLA."

Both the North and South maintain large numbers of forces along the border. Abyei is seen by many analysts as a potential flashpoint for a return to conflict.