BAGHDAD, Iraq — Four American soldiers were killed and two others were wounded yesterday when a roadside bomb exploded near their vehicles in eastern Baghdad, the American military said.

The soldiers were returning from search operations when one roadside bomb detonated, then another. The second bomb caused the casualties, the military said.

An explosives disposal team that inspected the site after the blasts found an EFP, explosively formed projectile, the military said. EFPs are powerful, armor-piercing roadside bombs that the American military has said are likely manufactured and imported from Iran. The weapons have killed at least 170 American soldiers since 2005.

The military did not identify the soldiers killed, pending notification of family members.

Earlier, a car bomb ripped through a square in central Baghdad, killing at least eight people, including several Iraqi police officers manning a nearby checkpoint, officials said.

And a car bomb slammed into bus ferrying government employees in Iskandariyah, a Sunni insurgent stronghold south of Baghdad. At least four people were killed and 24 were injured, a provincial police spokesman said.

The incidents underscored recent appraisals of the city ‘s month-old security plan by American military officials, who this week credited the crackdown for a drop in the number of murders and executions while noting that car bombings reached a record high in February. At the same time, violence has risen in some neighboring provinces, prompting American and Iraqi officials to speculate that Sunni and Shiite outlaws have migrated to the outskirts of Baghdad.

American forces have been seeking out car bomb factories with “some success in finding some and taking them out,” Major General William Caldwell, the top U.S. military spokesman in Iraq, said at a briefing Wednesday.