BAGHDAD (Reuters) – Six U.S. soldiers were killed by a blast north of Baghdad on Monday, the U.S. military said on Tuesday, in one of the single deadliest ground attacks against American forces in Iraq in months.

Three other soldiers were wounded after an explosion went off near their vehicles in the province of Salahaddin, the military said in a statement, without giving details. Salahaddin is a Sunni Arab insurgent stronghold. More than 3,170 U.S. soldiers have died in Iraq since the U.S.-led invasion in March 2003.
Iraqi and U.S. forces are in the third week of a security crackdown in Baghdad aimed at stemming sectarian violence. U.S. military commanders have expressed concerns that insurgents could step up attacks outside Baghdad.

Iraqi and U.S. forces are in the third week of a security crackdown in Baghdad aimed at stemming sectarian violence.(Paragraph3). The crackdown word means ____