* Libya rebels seize border crossing -- witnesses
* Thousands of Libyans have fled violence in remote region
* Area populated by Berbers, distrusted by Tripoli
(Adds Libyan denial, hospital, colour)
By Tarek Amara
TUNIS, April 21 (Reuters) - Rebels fighting Muammar Gaddafi's forces in a mountainous region took control of the Libyan side of a border crossing with Tunisia on Thursday, waving the country's pre-Gaddafi flag, witnesses said.
Thirteen Libyan officers and soldiers, including a general, handed themselves over to the Tunisian military at the border, Tunisia's state news agency TAP reported, apparently seeking refuge after clashes with the insurgents.
Witnesses said some wounded Libyan soldiers were treated at a nearby Tunisian hospital.
In Tripoli, Libyan government spokesman Mussa Ibrahim denied that the insurgents had taken over the border post.
Thousands of Libyans fleeing worsening violence in the remote region known as Western Mountains have poured into southern Tunisia and the border town of Dehiba in recent days.
The violence in the sparsely populated area has received little of the international attention given to attacks on cities on the Libyan coast such as Misrata and Ajdabiyah.
"We see rebels who control the border crossing," a witness who gave his name as Ali told Reuters by phone from Dehiba.
He said there had been fierce fighting near the border, lasting until Thursday morning, and that dozens of Libyan soldiers had turned themselves over to the Tunisian army. The Defence Ministry was not immediately available for comment.
The post is the smaller of Tunisia's two border crossings with Libya; the main one is at Ras Jdir further north.
Another witness at the crossing said rebels were celebrating by shooting in the air and several waved the pre-Gaddafi green, black and red Libyan flag -- a symbol of the rebellion against his rule. Gunfire could be heard over the phone.
Chanting "God is greatest", dozens of Libyan refugees from a camp near Dehiba as well as some Tunisians joined in the rebels' celebrations, witnesses said.
TUNISIA "ALARMED" BY MORTARS
TAP said about the incident. "Libyan revolutionaries ... took control of the border gate of the Libyan side of Dehiba crossing after a heavy exchange of fire."
In Tripoli government spokesman Mussa Ibrahim said when asked whether rebels had taken over the crossing: "I deny this. They are present on the Tunisian side and we know this and we warned about this."
Towns in Western Mountains joined a wider revolt against Gaddafi's autocratic four-decade-old rule in February and are now facing government bombardment.
The area is populated by Berbers, a group ethnically distinct from most Libyans and traditionally viewed with suspicion by Gaddafi's government.
A rebel spokesman in the region told Reuters late on Wednesday that there were clashes in the town of Nalut.
"The Gaddafi forces are using Grad rockets and mortar rounds to attack Nalut. It's not an even battle," the spokesman, Abdulrahman, said, citing fellow fighters in Nalut.
Libyan officials deny attacking civilians, and describe rebels as armed criminal gangs and al Qaeda sympathisers who, they say, are trying to destroy the North African country.
"In reality they are hiding in some caves in the Western Mountains. They come down, strike here and there and flee," spokesman Ibrahim said. More than 100 "armed elements" in the area had handed in their weapons on Thursday, he added.
Earlier this week, a Libyan doctor helping refugees in a camp near Dehiba said witnesses had told him that Gaddafi's forces had stepped up shelling of Western Mountains towns.
The doctor cited them as saying that dozens of people -- both rebels and civilians -- had been killed in the last week.
It is difficult to independently verify accounts of events in this region and other parts of western Libya because journalists have little access to these areas.
People who arrived last week in Tunisia from Western Mountains told Reuters that pro-government forces were shelling homes, poisoning wells and threatening to rape women.
Several mortar bombs fired from Libya landed on the Tunisian side of the border earlier this week. TAP cited a Foreign Ministry source on Thursday as saying it had told Libyan authorities it was "extremely alarmed by this incident."
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