A representative of Iraq's top Shia Muslim cleric has been assassinated along with his son and four bodyguards.
Ayatollah Sistani is Iraq's most influential religious leader
The attack against Sheikh Mahmoud al-Madaini came after evening prayers on Wednesday in the town of Salman Pak.
He headed the office of Ayatollah Ali Sistani in the mainly Sunni Muslim town in the lawless zone south of Baghdad.
It is the latest of a string of attacks launched by mainly-Sunni insurgents to derail national elections scheduled for 30 January.
The US has acknowledged that the elections are "not going to be perfect".
Sheikh Madaini had already received death threats from opponents of the elections.
Ayatollah Sistani is not running for office, but he has given his approval to the United Iraqi Alliance, a coalition of mainly Shia political parties.
The BBC's Jim Muir, in the Iraqi capital Baghdad, says a feature of the mounting pre-election violence has been provocative attacks on Shia figures, including their leading candidate, Abdel Aziz Hakim, who recently survived an assassination attempt.
He adds that responsibility for some of these attacks has been openly admitted by the radical Sunni factions leading the insurgency, who aim is to disrupt the election process and apparently to stir up sectarian trouble between Shias and Sunnis.
The Shia leadership has urged its followers to exercise restraint.
INSURGENT VIOLENCE MOUNTS
11 Jan: 15 Iraqis killed in separate attacks
10 Jan: Baghdad deputy police chief shot dead
7 Jan: Seven US soldiers killed in Baghdad bomb
6 Jan: Bodies of 18 Iraqis contracted to work at US base found outside Mosul
5 Jan: At least 25 Iraqis killed in three attacks in central Iraq
4 Jan: Governor of Baghdad, 14 Iraqis and five US soldiers killed in separate attacks
3 Jan: More than 20 people killed across Iraq
2 Jan: At least 23 Iraqi soldiers killed by a car bomb in Balad
In Washington, White House spokesman Scott McClellan told reporters: "We all recognise that the election is not going to be perfect."
But he added: "We're going to do everything we can to help the Iraqi people and the interim government ensure as broad a participation as possible in this upcoming election."
US military commanders say security is poor in four of the 18 provinces in Iraq - Nineveh, Anbar, Salahadin and Baghdad.
Iraqi Prime Minister Iyad Allawi has admitted that violence may prevent the election from going ahead in some areas of the country.
In other violence, gunmen have killed six people and abducted a Turkish businessman in an ambush outside a Baghdad hotel.
Up to 10 gunmen opened fire on a minibus that had come to pick up the businessman - identified as Abdulkadir Tanrikulu - from the Bakhan Hotel.
The dead were local employees of Mr Tanrikulu who had come to collect him in the minibus.
An Egyptian oil company manager was also kidnapped in the northern city of Kirkuk. He was named as Sayed Abdel Khalek.
Gunmen kidnapped four young Kurds in a separate incident in Kirkuk, which lies about 250km (160 miles) north of Baghdad.
Scores of foreigners have been seized in Iraq since last year. Many have been ransomed, but some have been killed.
Meanwhile, in the Sunni city of Ramadi, gunmen stormed a bank and made off with $13.5m worth of Iraqi dinars.
Ramadi is one of several Iraqi cities dominated by Sunni insurgents fighting US-led forces and Iraq's US-backed government.