A helicopter crash in northern Iraq on Wednesday killed 14 U.S. soldiers, the U.S. military said, the worst incident of its kind in more than two years.
Also in northern Iraq, at least 20 people were killed when a suicide bomber rammed a fuel tanker into the gates outside a police station in the oil city of Baiji, 180 km (110 miles) north of Baghdad. Another 40 people were wounded.
The police had just moved into new headquarters, situated among shops and houses, after a similar attack on its old building in June killed 27 people.
A U.S. military statement said initial indications suggested the Black Hawk helicopter had suffered mechanical failure, the second incident of its kind in eight days.
"There were no indications of hostile fire," it said.
"Two UH-60 Black Hawk helicopters were on a night operation when one of the aircraft crashed. That helicopter had been carrying four crew members and 10 passengers," it said.
The exact location of the crash was not immediately clear.
The losses take to 3,721 the number of U.S. military killed in Iraq since the 2003 invasion to topple Saddam Hussein. A total of 63 have died so far in August.
The latest crash was the worst since January 2005, when 31 service personnel were killed when a Marine transport helicopter was downed.
The Brookings Institution's Iraq Index says 67 U.S. helicopters have been downed since May 2003, 36 of them by hostile fire.
Five Americans were killed when a military transport helicopter crashed during a maintenance test flight west of Baghdad on Aug. 14.
The U.S. military has launched a nationwide offensive targeting Sunni Islamist al Qaeda fighters and Shi'ite militias to thwart an expected increase in attacks ahead of a key report on Iraq being presented to the U.S. Congress.
U.S. ambassador Ryan Crocker and General David Petraeus, commander of U.S. forces in Iraq, are due to deliver the report in September. It is widely seen as a watershed that could trigger a change in U.S. policy in Iraq.
Washington says the offensive is meant to buy time for Iraq's fractured Shi'ite-led government to reconcile Iraq's Shi'ite majority and minority Sunni Arabs, who are locked in a bitter sectarian conflict that has killed tens of thousands.
Pressure is growing on U.S. President George W. Bush to show results in the unpopular war or start bringing U.S. troops home, but Crocker on Tuesday described the Iraqi government's progress towards national reconcliation as "extremely disappointing".
