The three suspects appeared at the German Federal Court in southwestern city Karlsruhe on Saturday [GALLO/GETTY]

German law-enforcement officials have said three suspected members of al-Qaeda who were arrested on Friday, were planning a bombing.

Two of the men were detained in the western city of Dusseldorf and one in nearby Bochum for posing a "concrete and imminent danger" to the nation.

The three, who are said to be of Moroccan origin, appeared at the German Federal Court in the southwestern city of Karlsruhe on Saturday for a hearing in front of an investigating judge.

The suspects were experimenting with bomb-making though they had not picked any specific targets, officials said on Saturday at a news conference in Karlsruhe, a city in the southwest.

One suspect mentioned he wanted to "do a bus" and another had trained at a fighters' camp in Waziristan near the Afghanistan-Pakistan border, they said.

In a statement, Hans-Peter Friedrich, the German interior minister, said the arrests "succeeded in averting a concrete and imminent danger, presented by international terrorism".

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They showed "Germany remains a target of international terrorists," he said.

There was no indication that Friday's arrests had any link to Thursday's bomb attack in Morocco that killed 16 people in a crowded tourist cafe.

Germany has escaped any large-scale attack by extremists, such as the Madrid train bombings of 2004 and the London transit attacks of 2005.

But the country's presence as part of the NATO-led International Security Assistance Force in Afghanistan has sparked anger and at least two major plots have been thwarted or failed in Germany before they could be carried out.

The suspects had been under surveillance since November when Germany increased security across the country in response to heightened terrorism threat warnings in Europe, but authorities only had enough evidence to launch an official criminal investigation starting April 15, Friedrich said.

Federal prosecutors said earlier that they had ordered Germany's federal police to arrest the trio, but gave no further information about the timing or location of the arrests.

Duesseldorf, a city of 600,000 people, has one of the largest Moroccan immigrant communities in Germany.

Germany increased security in November after receiving information from its own and foreign intelligence services that led authorities to believe a sleeper cell of about 20 to 25 people may have been planning an attack inside the country or in another European nation.

Around the same time Germany also received information from US sources that an attack similar to that in Mumbai in November 2008 that killed 166 people, may have been planned for Germany.

Later, Germany received information on possible attacks at Christmas or New Year.

Source: Agencies