Twelve Palestinians were killed Tuesday as Hamas security police clashed with gunmen in the Gaza Strip, a Hamas security source said.

Among those injured in the exchanges of fire was Army of Islam leader Mumtaz Doghmush, who was involved in kidnapping of IDF soldier Gilad Shalit. His brother was killed.

The fighting between the Islamist Hamas security forces and members of the Doghmosh clan was the worst among Palestinians in the coastal territory since clashes in July in which more than a dozen died.

Hamas forces, responding to the killing of one of their policemen during an arrest raid on Monday, raided a clan stronghold in Gaza City before dawn on Tuesday in search of a suspect in that slaying, officials said.

Ten clan members, including an infant, and a Hamas operative were killed in ensuing clashes that went on for hours and continued into Tuesday morning. A Palestinian residing in the area was injured in the clashes and died of his wounds on Tuesday afternoon. Dozens of people were injured on both sides.

Others clan members were placed under arrest, the Hamas source said. The sounds of gunfire and explosions echoed through the city.

The exchanges of fire included the use of mortar shells, and according to one report one of the mortars landed near the home of senior Hamas member Mahmoud al-Zahar. This report has yet to be confirmed by Hamas officials or by the movement's security officers.

Following hours of battle, the Hamas interior ministry announced that the raid had been successfully completed and that the three wanted suspects were killed. Eyewitnesses reported seeing Hamas policemen continue to comb the area.

According to one report, the infant killed in the operation is the daughter of Zakaria Doghmush, secretary-general of the Hamas-affiliated Popular Resistance Committees organization.

Some clan members, allied with al-Qaeda, were involved in the March 2007 abduction of BBC reporter Alan Johnston, who was held hostage for four months before being released.

Others in the clan are divided between supporters of Islamist Hamas and those who back Palestinian President Mahmoud Abbas' Fatah movement, whose fighters were defeated when Hamas seized control of Gaza last year.