The discovery of the device sparked fears that Germany had become the target of an attack plot [Reuters]

A suspected bomb found in a luggage meant for a German-bound plane from Namibia has turned out to be a device used to test security checks, Germany's interior minister said.

Thomas de Maiziere said on Friday that the luggage was 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, but added it was not clear who had carried out the test.

The package, found at Windhoek airport on Wednesday, sparked fears of an attack in Germany and came on the same day that Berlin raised its national security level, boosting police presence at airports, train stations and public spaces.

De Maiziere said it was unlikely that the package was part of a German security test but the matter was being investigated.

'No explosives'

Namibian police found the package in a suitcase at the airport during the loading of an Air Berlin flight to Munich on Wednesday.

Germany's Federal Crime Office (BKA) said the package, found by Namibian police, contained "batteries that were attached with wires to a detonator and a ticking clock".

The interior minister said no explosives were found in the luggage and "there was at no point a danger to passengers posed by this luggage".

Air Berlin, Germany's second biggest airline, has made clear that the package was not found on its flight from Windhoek but inside the airport terminal.

Concern about the possibility of international flights being targeted by terrorists rose last month when two mail bombs were discovered while being sent on cargo planes from Yemen to the US.

The development came as the German government raised security measures following a tip-off from a "foreign partner" about an attack plot.

German authorities said on Thursday that they were on guard against threats of armed attack on civilians of the kind that killed 166 people in the Indian city of Mumbai in 2008.

"What we are basically preparing ourselves for, is that terrorists, coming from abroad, commit an attack soon after arrival, without warning, in a building or public place, knowing that they may not survive," de Maiziere told the German broadcaster ZDF.

"In shop talk, we'd call that something similar to what happened in Mumbai."

Citing security sources, the Tagesspiegel newspaper reported that the US had told Berlin that between two and four al-Qaeda operatives were on their way to Germany and Britain to attempt attacks.

Germany, which opposed the 2003 US-led invasion of Iraq but has nearly 5,000 troops in Afghanistan under Nato command, has never experienced an attack by al-Qaeda-linked operatives on its soil.