A suicide bomber killed 14 people lining up outside an Iraqi police recruitment centre northeast of Baghdad on Tuesday.
More than 23 people were wounded in the blast in the majority Sunni Muslim town of Muqdadiya, 90 km from the capital.
Police said the bomber was wearing a belt filled with explosives and most of the victims were people seeking to join the police force.
Four US soldiers were killed on Monday and April is on course to be one of the deadliest for troops in many months as more American and Iraqi forces deploy under a new security plan.
The latest deaths bring to around 45 the number of US soldiers killed in Iraq this month, half of them in the Baghdad area alone.
President George W Bush is sending 30,000 additional American soldiers to Iraq to bolster an offensive against militants in Baghdad that many regard as a last ditch attempt to halt Iraq's spiral into all-out sectarian war.
A key element of Operation Imposing Law is getting more US troops on the streets and assigned to dozens of joint security stations with Iraqi forces across the capital.
Three of the US soldiers were killed and another was wounded when a roadside bomb exploded near their patrol in Baghdad on Monday.
Another was killed in volatile western Anbar province, heart of the Sunni Arab insurgency.
The US military acknowledges the Baghdad security plan has increased the likelihood of more troop deaths.
"With more troops on the streets, there is more chance of casualties," said Lieutenant-Colonel Josslyn Aberle, a spokeswoman for US forces in Iraq.
The US military said a helicopter had taken small arms fire in Baghdad on Tuesday but denied reports from witnesses that one had come down in the capital.
Another US military spokesman, Major Steven Lamb, said a rocket pod on a helicopter had caught fire and was jettisoned. He had no further details.
The US military has said violence has dropped in Baghdad under the new offensive, with a 26 per cent decline in "murders and executions" between February and March, and a 60 percent fall between the last week of March and the first week of April.
But military commanders have stressed there is a long way to go before Baghdad is stabilised.
Around half the number of US troop reinforcements have arrived. All will be in place by end May or early June.
More than 3,280 American soldiers have been killed in Iraq since the 2003 invasion. Tens of thousands of Iraqis have been killed.