A suicide bomber has struck at an Iraqi security checkpoint in the northern Diyala province, killing at least 28 people and wounding 42 others, police say.

The bomber on Tuesday, wearing an explosives vest, blew himself up in a crowd of Iraqi police recruits in the town of Jalawla.

As other parts of the country have stabilised over the last year, Diyala - both ethnically and religiously mixed - has remained one of the most violent parts of the country.

US and Iraqi forces have been carrying out an offensive against armed groups there for the past month, saying that al-Qaeda in Iraq has regrouped in the area after being pushed out of other parts of Iraq.

In another attack, a blast in the northern city of Tikrit has killed at least four people and wounded six more.

A police official says the bomb was planted in a car parked on a street used by local government officials going to work.