Kofi Annan's special representative "expressed his serious concern at reports of grave, election-related violence" ahead of next week's polls, in a UN statement.

Citing incidents in Dohuk -- Iraqi Kurdistan -- and the Shiite holy city of Najaf, south of Baghdad, Qazi "underlined the responsibility of all political leaders to ensure that their supporters and activists exhibit the necessary restraint during the election campaign".

Three Iraqis, including an Kurdish electoral candidate, were shot dead in attacks against Islamic Union of Kurdistan party offices across the northern Dohuk province late Tuesday.

Assailants also fired a rocket against the party office of former Iraqi prime minister Iyad Allawi in Najaf on the same day.

"A respect for the ability of others to express different views, to organize and to argue their case in a peaceful manner is a fundamental requirement for building a new united, democratic, stable and prosperous Iraq," Qazi said.

The UN official also urged the Iraqi electoral authorities to ensure that the December 15 vote is conducted freely, fairly and transparently.

Around 15.5 million Iraqis are eligible to vote to elect 275 representatives to the country's first full-term parliament since Saddam Hussein was ousted by the US-led invasion in 2003.