Fighting has erupted along a new front near the internal border between north and south Sudan with dozens of people reported killed, aid workers and a U.N. report said.
The violence in South Kordofan has killed at least 25 people, according to a U.N. humanitarian report released Wednesday, though it said local sources indicated that up to 64 people had been killed. The north also bombed an airstrip, preventing the movement of food aid and humanitarian workers.
Fighting also broke out Wednesday in Abyei, another hot-button border region that the northern military invaded in May. A southern military spokesman said Wednesday's clash resulted in casualties but that he didn't have exact figures.
The increased violence comes less than a month before Southern Sudan declares independence from the north on July 9, the culmination of a 2005 peace deal that ended more than two decades of civil war that killed some 2 million people.
The sudden outbreak in violence on multiple fronts has greatly increased fears of renewed war, with some aid workers in the south indicating the northern government of Khartoum may be moving toward wider conflict.
The violence prompted U.S. President Barack Obama to issue a new warning to Sudan's leaders. He singled out the Khartoum government, saying it "must prevent a further escalation of this crisis by ceasing its military actions immediately, including aerial bombardments, forced displacements and campaigns of intimidation."
Obama said that if Sudan's leaders choose peace, the U.S. will take steps to normalize U.S.-Sudan relations, but that Khartoum will face more pressure and isolation if leaders there "flout their international obligations."
MiG fighter jets made multiple bombing runs over South Kordofan on Tuesday, according to accounts from international officials in the region. Vivid pictures showed a bomb exploding on an airstrip next to a U.N. compound in Kauda, a town in northern Sudan whose residents strongly support the south.
A U.N. spokeswoman, Hua Jiang, said 11 bombs were dropped in South Kordofan, five of which exploded. Two people were slightly wounded after the airstrip bombing, she said. Fighting was also heavy in the state capital, Kadugli, where violence broke out the first week of June.
"There have been some artillery shellings and small arms firing near Kadugli town and certainly the fighting since is moving closer to our headquarters in Kadugli," she said.
South Kordofan is part of northern Sudan, but many of the inhabitants there, the Nuba Mountain people, are black Africans who support the Christian and animist south against the Arab and Muslim-dominated north.
Jiang said 60,000 residents have been displaced by the violence, and that the U.N. is providing food and water to about 40,000 of them. However, the U.N. has been unable to fly in supplies for days. The U.N. refugee agency UNHCR accused Sudan on Tuesday of blocking aid deliveries in South Kordofan by air and road. Jiang said Wednesday that a road was opened to the U.N. on Tuesday.
The U.N. humanitarian report said it appears that southern troops are heading north toward Kadugli, feeding a "growing sense of panic among some of the displaced populations who find themselves trapped by the ongoing violence and the ethnic fault lines." Reports of sectarian violence against civilians targeting members of the different ethnic groups as well as wide spread looting of property are inhibiting returns to villages and towns of origin, even after the fighting has ceased."
In New York, U.N. Secretary-General Ban Ki-moon urged an immediate end to the violence so U.N. and humanitarian workers can drive and fly into Southern Kordofan to provide aid, spokesman Martin Nesirky said Wednesday. Ban is "deeply concerned" about the deterioration of the security situation there, he said.
An aid worker in South Kordofan who could not be identified because of security concerns said that four MiG jets and another large plane carried out multiple strikes on Tuesday, destroying the airfield at Kauda. The aid worker said it appears it will be impossible for any planes to land at the field, meaning no supplies can be flown in and no aid workers moved in or out.
Eric Reeves, a professor at Smith College in Massachusetts who has written extensively on Sudan, called the strikes "indisputable evidence of an attack on humanitarian efforts." He noted that the southern military and Nuba people have no military aircraft.
The aid worker's message said that people are "desperate."
"Individual(s) who contacted me said that what is direly needed is a no-fly zone. Really fear that these attacks are just the beginning of a long-term war that the (northern government) is going to carry out against South Kordofan the same way it has in Darfur over the past eight years," the aid worker wrote.
Meanwhile, in Abyei, troops from the north and south clashed. Casualties were reported after the clash near what southerners call the Kiir River, but southern spokesman Col. Philip Aguer said he didn't immediately have an exact toll.
Abyei _ a fertile land near oil fields _ is the major flashpoint between the north and the south. The situation spiraled out of control in May when the north invaded. Despite international calls for a withdrawal, northern troops remain in the region.
Aguer said the northern troops tried to cross the river on Wednesday. The U.N. spokeswoman said there were conflicting reports indicating that either northern troops or southern troops tried to cross.
Even as the violence intensified, northern and southern officials continued to meet in Ethiopia in hopes of finding peace.
A Southern Sudanese minister said Sudanese President Omar al-Bashir's troops should unconditionally withdraw from Abyei and allow an Ethiopian peacekeeping mission to move in with a mandate to defend themselves and civilians.
Deng Alor Kuol, the minister for regional cooperation for Southern Sudan, said African Union talks are focusing on empowering an Ethiopian peacekeeping mission for Abyei.
A cease-fire in South Kordofan seems far away. Deng said there are political issues to be addressed first with Abdul Aziz Al Hilu, the leading pro-southern political figure in South Kordofan, before any agreement that would be "effective" and accepted on the ground.
Associated Press reporters Luc van Kemenade in Addis Ababa, Ethiopia, and Maggie Fick in Seattle, Washington contributed to this report.