* Exodus from remote Libyan region into Tunisia
* Doctor cites refugees as saying dozens killed
(Recasts, adds quotes, background)
TUNIS, April 19 (Reuters) - Four mortar shells fired from Libya fell across the border in Tunisia this week, the latest sign of worsening violence in a remote Libyan region where residents say Muammar Gaddafi's forces are fighting rebels.
Thousands of people fleeing Libya's Western Mountains -- where some towns joined a wider revolt against Gaddafi's rule in February and are now facing an onslaught by government troops -- have poured into neighbouring Tunisia in recent days.
The state news agency TAP, citing Tunisia's Defence Ministry, said the mortar shells landed in an isolated area near the southern border town of Dehiba on Monday. No one was wounded and it was unclear who fired them.
A Libyan doctor helping refugees in a camp near Dehiba said witnesses had told him that Gaddafi's forces had stepped up their bombardment of towns in the Western Mountains.
The doctor, who gave his first name as Abdelrahman, also cited them as saying that dozens of people -- both rebels and civilians -- had been killed in the last week.
It is difficult independently to verify accounts of events in this region and other parts of western Libya, where journalists have little access.
The violence in sparsely populated Western Mountains has received little of the international attention given to attacks on cities on the coast such as Misrata and Ajdabiyah.
It is populated by Berbers, a group ethnically distinct from most Libyans and traditionally viewed with suspicion by Gaddafi's government.
Libyan officials deny attacking civilians, and say they are waging a battle against armed criminal gangs and al Qaeda sympathisers who, they say, are trying to destroy the country.
Abdelrahman, citing witnesses, said there had been fierce battles between government forces and insurgents since Friday in the regional towns of Nalut, Yafran and Zintan.
"According to witness accounts I heard in the refugee camp, dozens of people -- civilians and rebels -- were killed over the last seven days," he said. "Most were killed in the last three days when the attacks intensified."
Abdelrahman, declining to give his last name, said he worked and lived abroad but was in Tunisia to help fellow Libyans.
Isaa, a refugee from Nalut who spoke to Reuters on Abdelrahman's mobile phone, said homes had been destroyed and livestock killed in the shelling. "We heard that four people were killed in shelling attacks on Nalut today," he said.
People who arrived last week in Tunisia from Western Mountains told Reuters that Gaddafi's forces were shelling homes, poisoning wells and threatening to rape women in the region. (Reporting by Tarek Amara and Joseph Nasr; Writing by Marie-Louise Gumuchian and Fredrik Dahl; Editing by Tim Pearce)