Meanwhile, in Baghdad, a bomb planted in a parked car killed five people and wounded 20 more in the Shia-dominated Sadr City area.
On August 11, the governor and police chief of Qadasiya, a province in southern Iraq, died in a roadside bomb attack.
The governors killed in the two attacks were members of the Supreme Islamic Iraqi Council (SIIC), a group led by Abdul-Aziz al-Hakim, a Shia Muslim politician.
Supporters of the council have fought the Mahdi army, created by Muqtada al-Sadr, a Shia leader, for control of Iraq's oil-rich south.
Shia and Sunni groups have been fought sectarian battles in Iraq since the US-led invasion of March 2003.
However, recent months have seen intra-Shia violence between the Badr Organisation, the SIIC's armed wing, and the Mahdi army.
"This is part of a settling of scores prior to the elections next year," said a senior Shia official who declined to be named.
"I don't think there will be a Shia bloodbath because a decision has been taken to act with restraint. But more assassinations of some figures are expected," he said.
Riyadh Majeedh, Samawa's acting mayor, said Iraqi security forces had been deployed in the city and that an indefinite curfew had been imposed.
Hoda Abdel Hamid, Al Jazeera's Iraq analyst, said the successive attacks against provincial governors signified a marked rise in Shia-on-Shia violence.
| The Sadr City bomb killed five people |
and wounded 20 [AFP]
"The significance [of this particular attack] is in the succession of attacks against governors. I think it is showing that there is intra-Shia fighting that is getting bigger and bolder every day."
"We have seen on the ground already the Badr brigade, which is really the police force in southern Iraq, fighting with the Mahdi army."
Fighting broke out between various Shia factions in Samawa, which is about 370km southeast of the capital Baghdad, in July.
Abdel Hamid said the intra-Shia violence was in contrast to Washington's tradtional assessment that al-Qaeda is the biggest cause of destabilsation in Iraq.
"There is a lot of factional fighting that is nothing to do with al-Qaeda. The national intelligence estimate in the US has actually pinpointed al-Qaeda as the fifth threat to Iraq's stability rather than the first threat," she said.
"What is really destabilising Iraq now is factional fighting ... al-Qaeda is behind about 15 per cent of the attacks over the first six months of the year."
Al-Muthana was the first province that was transferred to Iraqi control by the British army last year.
Amid the growing discord between Shias in Iraq, al-Sadr said al-Maliki's government was close to collapse despite efforts to bolster it.
"Al-Maliki's government will not survive because he has proven that he will not work with important elements of the Iraqi people," al-Sadr said in an interview published by London's Independent newspaper on Monday.
"The prime minister is a tool for the Americans, and people see that clearly," he said in the interview, which was conducted in the southern Iraqi city of Kufa.
"It will probably be the Americans who decide to change him when they realise he has failed. We don't have a democracy here, we have a foreign occupation."
Al-Sadr, who formerly expressed support for al-Maliki, withdrew his five supporters from the Iraqi cabinet to protest the prime minister's refusal to demand a timetable for the pullout of US forces from Iraq.