Berlin (AFP) - Two German-speaking jihadists claiming to belong to the Islamic State militant group threatened Germany with attacks in an execution video broadcast online Wednesday.

The five-minute clip shows the men use assault rifles to kill two bound and kneeling male hostages in the Syrian ancient city of Palmyra, which IS fighters took in May.

In a rare German-language IS message, they urge their "brothers and sisters" in Germany and Austria to join the IS in Syria and Iraq or commit attacks against "unbelievers" at home.

One of the men threatens Chancellor Angela Merkel, vowing to avenge the "blood of Muslims spilled in Afghanistan", where Germany sent troops as part of a NATO force, and their support for the coalition against IS.

German intelligence estimates 600 Germans have joined jihadist groups in Syria and Iraq.