The United States has lost more than 400 military drones in major crashes worldwide since 2001, The Washington Post said in a report questioning the safety and reliability of the unmanned aircraft.

Citing 50,000 pages of accident investigation reports, the Post said that military drones had since the 9/11 attacks "malfunctioned in myriad ways," including mechanical breakdowns, human error and foul weather.

"Military drones have slammed into homes, farms, runways, highways, waterways and, in one case, an Air Force C-130 Hercules transport plane in midair," the report said.

Of the 418 known crashes between September 11, 2001 and the end of 2013, the Post said it had identified 194 so-called Class A crashes that resulted in either the total loss of a drone or damages in excess of $2m.

The total figure is almost equal to the number of major crashes involving US Air Force fighter jets and attack planes during the same period - even though the drones flew far fewer missions and hours.

Sixty-seven drone crashes occurred in Afghanistan, and 41 in Iraq, but 47 occurred within the US during test and training flights, the Post said on its website.

The Post's investigation comes as the Federal Aviation Administration drafts a set of regulations to govern an expected surge in the use of commercial drones in the coming years.