A helicopter crash in northern Iraq on Wednesday killed 14 US soldiers, the US military said, the worst incident of its kind since January 2005.
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 volatile oil city of Baiji, 180 km (110 miles) north of Baghdad. Another 50 people were wounded.
A US 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.
Police had just moved into new headquarters, situated among shops and houses, in Baiji after a similar attack on their old building in June killed 27 people, including 13 policemen.
Abdul Rahman Mahmoud, a 22-year-old college student, said many of the victims were from a local education ministry office next door to the police directorate.
Other witnesses said part of the police building had collapsed and victims were trapped in the rubble. Many nearby shops were set ablaze.
Police fired their weapons into the air to disperse crowds as wounded were piled into cars and rushed to hospital.
The helicopter crash deaths take to 3,721 the number of US 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. In January, 12 soldiers died when a Black Hawk was shot down northeast of Baghdad.
Five Americans were killed when a military transport helicopter crashed during a maintenance test flight west of Baghdad on August 14.
The US 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 US Congress.
Iraqi Interior Ministry spokesman Abdul Karim Khalaf said Iraqi security forces had killed 56 suspected Al- Qaeda members during an operation in Fadhil, a notorious insurgency stronghold in central Baghdad, on Tuesday.
Fadhil residents said there had been intermittent fighting between al Qaeda and other Sunni Arab militant groups over the past two days. One resident said he had seen four bodies and many wounded being taken away in ambulances.
US ambassador Ryan Crocker and General David Petraeus, commander of US forces in Iraq, are due to deliver their progress report in September. It is widely seen as a watershed that could trigger a change in US policy in Iraq.
Washington says US troops are fighting to buy time for the 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 US President George W. Bush to show results in the unpopular war or start bringing US troops home, but Crocker on Tuesday described the Iraqi government's progress towards national reconciliation as "extremely disappointing".
Speaking in Ottawa on Tuesday, President Bush urged Prime Minister Nuri al-Maliki to do more to broker peace between the warring sides. He said it was up to the Iraqi people whether to replace the government.
On a visit to Damascus on Wednesday, Maliki hit back at US criticism, saying nobody had the right to set timetables for progress.