KABUL, Nov 9 (Reuters) - Rival groups of Taliban militants have clashed in the southern Afghan province of Zabul, killing as many as 80 people in recent days, officials said on Monday, as brewing hostility between factions in the insurgency turned violent.

Government officials and spokesmen for the two main Taliban groups said fierce fighting had been underway since the weekend, with each side blaming the other for starting the violence.

Insurgents who have pledged allegiance to Islamic State may also have been involved.

Seven members of the mainly Shia Hazara ethnic group, including three women and a child, were found with their throats cut on Sunday in an incident police blamed on Islamic State militants.

The fighting, in one of the Taliban's traditional southern strongholds, underlined the risk of fragmentation facing the Islamist movement since it announced earlier this year that its founder, Mullah Omar, had died two years ago.

A prolonged split in the movement could further complicate the resumption of peace talks with the government, which broke down in July following the announcement of Mullah Omar's death.

One of Mullah Omar's close aides, Mullah Akhtar Mansour, was swiftly appointed as successor, but his claim to the leadership has been rejected by some in the movement and a splinter faction named rival leader Mullah Mohammad Rasool Akhund last week.

"This is what we feared," said a senior member of the main faction led by Mullah Mansour, who spoke on condition of anonymity. "The enemy succeeded in their mission. They wanted Taliban to be divided and kill each other and that's exactly what's going on in Zabul now."

He said Islamic State militants were supporting the splinter group, which was also backed by about 400 Uzbek militants closely allied to Mullah Mansoor Dadullah, one of the leaders of the breakaway faction.

"Most of these Uzbek fighters are affiliated with IS in Afghanistan," he said.

However, a spokesman for the anti-Mansour faction, Mullah Abdul Manan Niazi, said the battles broke out when hundreds of pro-Mansour fighters attacked villages where Mansoor Dadullah's followers were living.

"Mullah Mansour's men wanted Mansoor Dadullah to surrender or face action. When they refused to surrender, Mullah Mansour's fighters started killing them and their supporters as well as local villagers," he said.

It was not possible to verify the competing versions independently, but provincial officials said fighting between the rival groups had been going on for the past three days in Khak-e-Afghan, Daichupan and Nawbahar districts of Zabul province.

Adding to the confusion, Mohammad Omar Omari, a member of the provincial council in Zabul, said local people outraged at the killing of the seven people whose bodies were found a day earlier had joined the fighting.

He said they had seized 12 Islamic State fighters in Khak-e-Afghan district and hanged them for the killing. (Reporting by Jibran Ahmad and Aimal Yaqubi; Writing by James Mackenzie; Editing by Mike Collett-White)

Our Standards: The Thomson Reuters Trust Principles.