Two bombers detonated their four-wheel-drive vehicle packed with explosives outside a US military base north of Baghdad in the Sunni heartland, where loyalty to deposed dictator Saddam Hussein still runs strong.

The blast near Balad, 60 km (36 miles) north of the capital, was the latest of many attacks on Iraq's fledgling security forces in a bloody campaign to scare voters away from the polls.

Twenty-one National Guards and one Iraqi civilian were killed and half a dozen people wounded, Iraqi police said.

US and Iraqi officials ushered in the New Year with warnings of an expected spike in pre-election assaults by insurgents trying to drive out US-led forces and topple Iraq's American-backed government.

"Those responsible for this attack ... are trying to prevent democracy in Iraq," said Major Neal O'Brien, a military spokesman in Tikrit. "They will not be successful."

On Saturday, the al-Qaeda Organisation of Holy War in Iraq led by Jordanian al-Qaeda ally Abu Musab al-Zarqawi released a video of five Iraqi security men being shot dead in the street.

A statement posted on an Islamist Web site along with the video vowed that the group would "slaughter" other Iraqis it brands collaborators with foreign occupiers.

Yesterday's attack was the deadliest suicide bombing against Iraqi security services since mid-September, when at least 47 people were killed outside a Baghdad police station.

Guerrillas have killed hundreds of security force members since the US-led invasion in 2003. Many Iraqis wonder how police and National Guards will be able to protect voters when they can barely protect themselves.

Insurgents assassinated two local government officials for Diyala province, northeast of Baghdad, and an Iraqi police major outside his home in Baghdad on Saturday, signalling they would persist with their campaign in the new year.

Zarqawi's group claimed responsibility for one of the deaths, that of Nawfal Abdul-Hussein al-Shimari, head of Diyala's governing council.

GUARDS SHOT DEAD
In the al-Qaeda-linked group's video, masked militants lined up five captive National Guardsmen, their hands bound behind their backs, and shot them from behind.

Some passers-by were seen stopping to watch.

"To the families of civil defense forces, the National Guard and the police we tell you to say your final goodbyes to your sons before you send them to us. Our reward to your sons is slaughter," a masked militant said in a statement.

Five men in civilian clothes were found shot dead in Ramadi, capital of restive Anbar province, earlier this week. A note said they were security men killed by guerrilla fighters.

On the video, one of the captured men who identified himself as Lieutenant Bashar Latif Jasem said his duty was to fight "terrorists entering Iraq."

When the shooting begins, the men fall to the ground but gunmen keep pumping bullets into them.

Zarqawi's group has claimed most of the bloodiest suicide attacks in Iraq since the fall of Saddam and has beheaded several foreign hostages and led a campaign of assassinations, bombings and ambushes against Iraq's new security forces.

Iraqi interim Prime Minister Iyad Allawi told Iraqis in a New Year's eve broadcast on state television his government would do all it could to ensure voters' safety, backed up by US-led troops and the new Iraqi security services.

Osama bin Laden and Islamist groups have pledged to wreck the vote as part of a holy war in Iraq.