Sudan's military is carrying out a bombing campaign intended to shut down the main route for refugees fleeing violence in the country's south, a rebel spokesman said Monday.

A former American aid worker who lives in the region documented five attacks and clashes last week.

Attacks by the Sudanese Armed Forces have been focused along the road leading out of Southern Kordofan, Sudan, into Yida, South Sudan, said Arnu Loddi, a spokesman for the Sudan People's Liberation Movement-North, a rebel group inside Sudan.

More than 20,000 refugees have already fled to a camp in Yida to escape the violence, and the United Nations worries that hundreds of thousands more could be on the way.

According to Loddi, Sudanese Armed Forces have been launching missiles from a military base in Kadugli, Sudan into the border region. He said Sudanese forces also launched "an ambush on a lorry carrying civilians going to Yida," last week. Loddi did not have any information on the number of casualties sustained in the attack.

A Sudanese military spokesman did not immediately answer calls seeking comment about the reports.

"They want to close down the roads," said Loddi, who is based in Nairobi, Kenya. "Sometimes they ambush, sometimes they bomb it. Since the beginning of this year they have bombed the same area at least five times."

The rebel SPLM-North was once part of the Southern People's Liberation Movement, now the ruling political party in newly independent South Sudan.

The SPLM says it cut off relations with the rebels across the border after South Sudan broke away from Khartoum in July. But Sudan claims that the SPLM-North still receives funding and support from Juba and even maintains a presence in refugee camps -- including Yida -- across the border.

Ryan Boyette, a former aid worker who lives in Sudan and is now leading a team of 15 citizen journalists, said that Sudanese forces carried out at least five attacks last week, including rocket attacks, aerial bombings and a direct attack on a village by troops.

One of the attacks saw Sudanese Armed Forces directly engage South Sudan forces in Jau -- along the road to Yida -- Loddi said. Four southern troops were injured, he said.

Jau is claimed by both north and south, and the two sides have been on high alert since the attack. South Sudan's military spokesman Col. Philip Aguer said some 5,000 Sudanese troops have been moved to the region.

"We don't know their intention," he said. "We are just watching and waiting."

Aguer denied that any SPLM-North forces are operating inside South Sudan.

While the two sides jockey for control of Jau and the unmarked border, the humanitarian situation in Southern Kordofan is deteriorating. Sudan has been at war with the SPLM-North since June, frequently employing Antonov aircraft to bomb rebel controlled areas.

The repeated attacks have effectively halted agricultural production in the region, and many humanitarian organizations warn that food is beginning to run out in the Sudanese state of Southern Kordofan. Khartoum has refused to allow relief organizations into the area citing security concerns and fears that aid may be given to rebel fighters.

Last week the U.N.'s top humanitarian official, Valerie Amos, called on both Sudan and the SPLM-North to grant immediate humanitarian access to the region.

"Action is urgently needed to meet the needs of people caught up in the conflict," she said.