The US Air Force has offered $5,000 (€4,200) as a reward for help finding a container of grenades which fell off the back of a truck.

The grenades were lost as the 91st Missile Wing Security Forces team travelled between test sites in North Dakota's north west.

The back hatch of the team's humvee opened about six kilometres from Parshall and the container fell out, the air force said.

But although this happened on 1 May, the situation was not reported to the local sheriff until three days later.

According to a report in the Williston Herald, Mountrail County Sheriff Ken Halvorson said: "The lack of information being released by the Air Force on this loss and concern for the safety of the citizens in the area that may come into contact with (the ammunition) prompted the release of this information."

The air force said that the ammunition was safe as long as the container remained intact but Mr Halvorson added that the ammunition would not operate in another device without "catastrophic failure".

The search has since been called off by the air force, after more than 100 airmen from Minot Air Base walked the 10 kilometre route several times on Friday in an unsuccessful search for the grenades.

Residents, including farmers and oil workers, were warned to call emergency services if the ammunition was found.

As the explosives were lost on the Fort Berthold Reservation, Native American tribal staff were also warned.