Iraqi police say at least 28 people, including seven policemen, were killed in a powerful explosion in a Baghdad house overnight.

Officials describe the incident as an ambush, saying "massive amounts of explosives" were detonated as police raided the building. Police had earlier received an anonymous tip that the house was a suspected base for foreign militants.

Officials say several nearby houses also collapsed, and they fear a number of residents might be trapped under the rubble.

Tuesday, at least 23 Iraqi police and national guards were killed in a series of insurgent attacks in the so-called Sunni Triangle region north of Baghdad - in and near the towns of Tikrit, Samarra, Balad and Baquba.

Top U.S. military officials in Iraq say they expect an escalation of attacks and assassination attempts ahead of the January 30th elections.

Some information for this report provided by AP, AFP and Reuters.