BAGHDAD, Iraq - At least six police officers were killed - including Baghdad's deputy police chief and his son - in two separate attacks today, as insurgents stepped up their attacks three weeks ahead of the country's landmark elections.

Over the weekend, U.S. troops opened fire near a checkpoint south of Baghdad after their convoy was hit by a roadside bomb, and a hospital official said Sunday at least eight people were killed.

Toay, Brig. Amer Ali Nayef and his son, Lt. Khalid Amer, were killed by machine-gun fire sprayed from two cars driving parallel to the chief's as he and his son drove to work, police said.

In a separate attack, a suicide car bomb exploded in the courtyard of a police station in southern Baghdad today, killing at least four officers and injuring 10 others, police and witnesses said. A fake police car packed with explosives was used in the attack.

The explosion took place at 8 a.m. in the Zafarniyah district, police commissioner Abdul Khaleq Hussein said. Witnesses said the explosion happened as policemen were changing shifts. An Associated Press photographer saw a number of bodies inside the courtyard that was cordoned off by police.

Meanwhile, U.S. officials said they had no information about the checkpoint shooting, which occurred overnight Saturday. Interior Ministry spokesman Col. Adnan Abdul-Rahman said a roadside bomb hit a U.S. convoy near a police checkpoint in Yussifiyah, nine miles south of Baghdad, and troops opened fire, killing two police officers and three civilians.

American commanders recently announced a change in response to roadside bombings. Rather than pushing on after the blast, they now stop and try to engage the perpetrators, who may have detonated the explosives remotely.

In other violence Sunday, a U.S. soldier assigned to Task Force Baghdad was killed by a roadside bomb, and a Marine was killed in action in the volatile Anbar province.