Hamburg (dpa) - German prosecutors have opened an inquiry into claims that terrorists plotted to set off a truckload of explosives in Germany, the news magazine Der Spiegel reported.

The plot had involved driving a truck containing a ton of explosives to Germany via Russia and Finland and crossing the Baltic Sea by ferry to the port city of Rostock, said Spiegel.

German federal police had counselled federal agencies to take due precautions.

The magazine released its report two days in advance of Monday publication. Asked by DPA for confirmation, prosecutors and police declined comment.

In Beirut on Friday, a Lebanese security source told media an alleged al Qaeda figure suspected of involvement in attempted train bombings in Germany had been arrested in Lebanon after threatening to carry out attacks in Germany over the next three months.

Mohammed Ndoub, a Syrian national, was arrested after issuing threats to the German embassy in Beirut via a public telephone.

Ndoub had told the embassy he would carry out several attacks against civilian targets inside Germany in the coming three months.

The attacks would be "to avenge" the recent conviction in Beirut of one accused and the ongoing trial in Germany of the other for the failed July 31, 2006, bombings of two German passenger trains.

Another weekly, Focus, said Ndoub had claimed that a team of three men were already in Germany. The "Jihad Islami" team comprised a German of Turkish ethnicity, a Saudi national and an Australian. Focus said Lebanese police assessed the claims as broadly plausible.

Spiegel said the threat suggested the German armed forces, security agencies or justice ministry might be the targets.

Although such warnings in the past had always proved wrong, the tip-off did match recent speeches by al Qaeda deputy leader Aiman al-Sawahiri.

One would-be bomber Jihad Hamad was sentenced in Lebanon last month to 12 years imprisonment, while a second, Youssef al-Haj Dib, is currently on trial in Germany.

The two had placed two explosives-laden suitcases on the trains, which failed to detonate owing to technical faults.

German investigators said the planned explosions near Hamm and Koblenz would have resulted in the deaths of hundreds of people and possibly been on a larger scale than the July 2005 London transport attacks.

Spiegel said the threat was also linked to the German arrest last September of a trio of sympathizers with al Qaeda for plotting bomb attacks.