BAGHDAD - A suicide bomber killed at least six mourners at funeral for a member of a prominent tribe with ties to both security forces and insurgents in western Iraq on Monday, a police official said.

The bomber detonated an explosive belt inside a funeral tent in the mostly Sunni area of Haditha, about 220 kilometres northwest of Baghdad. At least 15 people were injured, said the official, who spoke on condition of anonymity because he was not authorized to speak to media.

The funeral was for a member of the al-Jaghaifa tribe, which is split between members of the police and military and supporters of the Sunni insurgency.

Haditha is located in Anbar Province, which was once a stronghold for the Sunni insurgency but became a key battleground for U.S.-led forces after many tribes turned against al Qaeda in Iraq and other groups.

In southern Iraq, meanwhile, the country's most senior Shiite cleric threatened to call for a boycott of January's parliamentary elections if the voting system includes only the parties and not the names of the candidates.

Iraqi lawmakers are considering having the Jan. 16 ballots list only the party blocs and not the individual candidates, but Grand Ayatollah Ali al-Sistani wants a so-called "open list" system because he believes it will encourage more voter participation.

An official in al-Sistani's office says the cleric may "not play a big role in the political process" -- which is widely interpreted as a warning he could call for a boycott. He spoke on condition of anonymity Monday because he was not authorized to talk to the media.

Such a call by al-Sistani would likely bring massive no-shows at the polls and raise questions about the legitimacy of the outcome.

Iraq's majority Shiites have been split into two political camps: one led by Shiite religious parties and the other a broad coalition put together by Prime Minister Nouri al-Maliki.