* Southern army says it clashed with northern troops
* Negotiators still in talks on Abyei in Addis Ababa
* Over 100,000 people have fled Abyei fighting - U.N.
(Updates with selling of town)
By Jeremy Clarke and Alex Dziadosz
JUBA/KHARTOUM, June 17 (Reuters) - Shelling erupted on Friday near the southern Sudanese town of Agok at the tip of the disputed Abyei region, and aid workers were being evacuated from the town, a humanitarian official said.
"This morning, from about 10 am, there was shelling on both sides of the river. Later there was shelling just a little bit north of Agok ... All humanitarians are being pulled out of Agok," the official, who asked not to be named, told Reuters.
A spokesman for the northern military was not immediately available to comment. The southern army's spokesman said northern and southern forces clashed in the area on Friday, but said he did not have details yet.
South Sudan is just over three weeks away from declaring independence, but Khartoum and Juba have yet to decide who should control the fertile, oil-producing Abyei region, raising fears the two sides could return to open conflict.
Khartoum seized Abyei's main town on May 21, pushing as far south as the Kiir River, known as Bahr al-Arab in the north, which has since become the front between the two sides. Over 100,000 people have fled the fighting, the United Nations says.
The southern army said it had clashed with northern troops in the area, but it was not clear if that was connected to the reported shelling near Agok.
The U.N. mission in Sudan (UNMIS) said the north's Sudanese Armed Forces (SAF) stated the shelling was part of an exercise.
"We confirm that six shells by SAF ... hit Agok, 150 metres from the UNMIS base," spokesman Kouider Zerrouk said, adding there were no reports of casualties.
"Both sides must stop military actions which are causing fear to the already affected populations in the area," he said.
The southern military said it exchanged fire with northern forces in Abyei on Wednesday -- an accusation the northern army denied, suggesting internal southern rebel militias may have been behind the clashes.
Southern army spokesman Philip Aguer said there had been more clashes on Friday, but did not give details.
"There was a clash in the same bridge area. The SAF (the northern Sudanese Armed Forces) came back. SPLA (the south's Sudan People's Liberation Army) fought them," he said. "We think the area might be overrun by SAF."
Negotiators from both sides have been meeting in Ethiopia since Sunday. Despite reaching an agreement "in principle" to demilitarise Abyei, no final deal has emerged yet.
COMPLICATED SPLIT
Southerners voted for independence in a January referendum promised in a 2005 peace agreement that ended a long and brutal civil war. About 2 million people died in that conflict.
A host of unresolved questions have since complicated the split, including where to place the common border and how to manage the country's oil industry after the secession.
Fierce fighting with artillery and warplanes in the north's volatile Southern Kordofan border state has worsened tensions over the past two weeks. Tens of thousands have fled the fighting there, the United Nations says.
The northern army detained four U.N. peacekeepers in Southern Kordofan's capital Kadugli on Thursday, a move the U.N. strongly condemned, UNMIS spokesman Zerrouk said.
"The SAF, SPLA and other armed groups in the area must immediately stop intimidating and harassing U.N. staff who are critical to providing necessary humanitarian assistance to the vulnerable populations," he said.
In a separate report, the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said a mission visit to the settlements of Wunrok, Mayan Abun, Turalei and Agok found staple food prices had gone up by 40 percent in local markets.
"This is a cause for concern, in particular as rains in the next fortnight are expected to intensify rendering access roads impassable to Agok and reducing grain supply to the market," the report added. (Additional reporting by Stephanie Nebehay in Geneva; writing by Alex Dziadosz; editing by Andrew Roche)
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