A suspected bomb intercepted in Namibia that was to be put on a Munich-bound charter plane was only a US-made dummy used to test security checks, Germany's interior minister said.

Thomas de Maiziere said it was not immediately clear who had carried out the test, which sparked a major security alert Wednesday.

"Experts from the [German] federal police force examined the luggage on site," De Maiziere told reporters after a security conference with interior ministers from Germany's 16 states.

"The outcome is that the luggage turned out to be a so-called real test suitcase made by a company in the United States. This company is a manufacturer of alarm and detection systems and these real test suitcases are built to test security measures."

He said investigators were still examining who placed the suitcase with baggage to be loaded on to an Air Berlin plane at the international airport of the Namibian capital Windhoek, including whether German security forces could have been involved in the test.

"I consider that highly unlikely but that is one of the things we are looking into," Mr De Maiziere said.

"The important thing for all of us is that no explosives were found in the luggage and that, as far as we know at this point in the investigation, there was at no point a danger to passengers posed by this luggage."

German federal police said that the suspicious baggage, a laptop bag wrapped in plastic, had been seized by Namibian police and that a subsequent X-ray revealed batteries that were attached with wires to a "detonator" and a ticking clock.

The find sparked a probe into whether the bag contained live explosives, one day after De Maiziere said that the security services had received a tip from a "foreign partner" about an attack planned in Germany in the next two weeks.

The Air Berlin plane with 296 passengers and 10 crew members on board was delayed six hours before being cleared for take-off to Munich, where it arrived safely early Thursday morning.

- AFP