At least 15 senior al-Qaeda members have escaped from a prison in Tikrit, Iraqi officials said, with authorities locking down the city as they search for fugitives.

Major-General Abdul-Karim Khalaf, an Iraqi military spokesman, said on Thursday that six of the escaped inmates are considered dangerous.

Police sources told Al Jazeera that all of the men are charged with killing civilians and attacking security forces in the province in Salahiddien, north of Baghdad.

They added that one of the prisoners, who has been sentenced to death, was re-arrested near the jail.

The prisoners are said to have escaped through a window space half a metre square.

The break-out is not the first time al-Qaeda members have escaped from jail, suggesting the prisoners received outside assistance.

Al-Qaeda are believed to pay up to $20,000 for someone who can help to free their senior leaders from jails.

A curfew was imposed on the city after the escape, which occurred on Wednesday night.

Authorities are distributing wanted posters with photos of the fugitives and are running checkpoints around Tikrit, the hometown of Saddam Hussein, the deposed Iraqi leader.

The city is about 130km north of Baghdad, the Iraqi capital.