A suspicious package found in a Namibian airport near bags bound for Munich was designed to test airport security, Germany's top security official has said.

But officials are still trying to determine who carried out the test.

Thomas de Maiziere said that German experts sent to Windhoek airport after the incident on Wednesday had inspected the bag and determined it was a case made by an American firm to test airport security measures.

He did not say who may have left the case in the luggage hall, adding that German agents were still investigating.

The threat came amid heightened concerns of terrorist threats against Germany that prompted Mr de Maiziere to raise the country's threat level even before the Windhoek incident.