Gunmen in Pakistan have killed three supporters of former prime minister Benazir Bhutto amid campaigning for January general elections, police say.

The attackers opened fire on an office of Bhutto's opposition Pakistan People's Party (PPP) in a village in the south-western Baluchistan province.

The pre-dawn attack could be linked to tribal rivalries, police said.

Violence is common in the run-up to and during elections in Pakistan, especially in remote rural areas.

The shooting occurred in the village in Naseerabad district, about 240km east of the provincial capital of Quetta, police said.

"Unidentified men entered the PPP office and sprayed bullets on men sleeping there, killing three and injuring another," a local police officer was quoted as saying by the news agency.

Police believe the attack could be linked to a land dispute between two local tribes.

The death toll was later confirmed by PPP officials.

Bhutto returned to Pakistan in October after spending eight years in exile.

Her homecoming was overshadowed by two bomb blasts in the southern city of Karachi that killed some 140 people celebrating her return.