"Germany is at the centre of al-Qaeda's attention and in their line of fire. The facts have changed since last year," said interior ministry spokesman Stefan Paris, confirming a press report.

A state secretary in the interior ministry, August Hanning, told Die Welt newspaper that al-Qaeda leaders based in the border area between Pakistan and Afghanistan had "decided to carry out attacks in Germany."

"We are worried that we will not be able to foil every plot," he added.

Die Welt said Germany's domestic intelligence agency and police had established that Germany's six-year-old military mission in Afghanistan had prompted al-Qaeda to move the country "much higher" on its list of targets.

Hanning linked the "high risk" to the volatile situation in southern Afghanistan, saying al-Qaeda's "operational capacity" in the region had recovered.

In September, two German converts to Islam and a Turkish man were arrested in the western Sauerland region on suspicion of planning to blow up US installations in Germany, including the southwestern US military airbase at Ramstein.

The men had stockpiled some 700 kilogrammes of chemicals to use in "massive" attacks to coincide with the anniversary of the September 11, 2001 suicide hijackings in the United States, the security services said.

The plotters are believed to belong to the Islamic Jihad Union, a group with links to al-Qaeda.