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 Houthis have waged a campaign against international shipping to show solidarity with Hamas, firing drones and missiles at ships in the Red Sea, a vital route for international trade.

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.