Yemen's Houthi rebels have threatened to hit vessels of German shipping firms passing near the Red Sea or using Israeli ports, the German shipowners' association VDR said Monday.
The emailed warnings sent to the German industry body and cargo carriers in recent months were "attempts at intimidation", VDR executive Irina Haesler said.
The threats were "directed against ships that call at Israeli ports, as well as against those that pass through the Red Sea, the Bab al-Mandab Strait, the Gulf of Aden, the Arabian Sea and the Indian Ocean", the association said.
"Regardless of their location, ships with supposed links to Israel are considered potential targets," it said.
One such email from the Houthis, seen by AFP, warned German shipowners of "a naval blockade on the Israeli enemy".
It said "all vessels belonging to it, associated with it or bound for it" would be "subject to punishment and ... prohibited from crossing the area of operations of the Yemeni Armed Forces".
A VDR spokeswoman told AFP the authenticity of the emails had been confirmed by the German navy and International Chamber of Shipping.