At least five policemen have been killed in a suicide car bomb attack at a checkpoint near the Iraqi capital, police say.

At least 13 other people were hurt in the blast on Saturday in Garma, which lies 30km northwest of Baghdad, police Lieutenant Mohammed al-Jumaili said.

The attack comes as Iraqis prepare to vote in provincial elections seen as a test of recent improvements in security, following years of ethnic violence sparked by the 2003 US-led invasion.

US officials say that there could be an increase in violence before the January 31 election, with fighters continuing to launch attacks across the country.

On Saturday, Iraqi election officials ordered transport bans and night-time curfews during the polls.

Iraq's borders will also be sealed off, while all civilian airports and provincial borders will be shut from 10pm (19:00 GMT) on Friday
until 5 am (02:00 GMT) on February 1, the election commission said.

"In no instance will any car be authorised to approach a polling station," it said.

Iraqi police say they have evidence to suggest that al-Qaeda fighters are still operating in Garma, which lies in the western desert province of Anbar.

The province was the main stronghold of al-Qaeda, a key player in the Sunni Muslim Arab resistance to the US invasion, but tribal chieftains have forced out al-Qaeda over the last two years.

Although Anbar is now calmer, analysts say that tensions between Sunnis are increasing in the run-up to the provincial elections.