A source in Germany's BfV domestic intelligence service said on Wednesday that Islamists could stage an attack in Germany at any time with the lethal toxin ricin.

A week ago, a 29-year-old Tunisian, named by authorities as Sief Allah H, was arrested in Cologne on suspicion of planning an Islamist-motivated attack and building a "biological weapon" using ricin.

The BfV source said Isil (Islamic State of Iraq and the Levant) militants had experimented with ricin in the past and had also manufactured it. Isil, the source added, had offered detailed instructions on how to produce ricin in a handbook.

Holger Muench, president of the Federal Criminal Office, told the broadcaster RBB-Inforadio that the suspect had manufactured ricin, a poison found in castor beans, and that objects had been found in searches that could be used to make a bomb.

He added that the target for the suspected attack was not clear.