Germany accused Turkey on Monday of preventing German forces belonging to an EU military mission from fully searching a Turkish cargo ship that they suspected of taking weapons to Libya, a move Ankara said was a violation of international law.
Soldiers from the frigate Hamburg boarded the Turkish freighter, the Roseline A, overnight, but had to abandon checks and withdraw after Turkey protested to the EU mission, which had ordered the search, the German Defence Ministry said.
Turkey said the search, at sea southwest of the Greek Peloponnese peninsula, violated international law as permission had not been granted.
The incident is further friction between Turkey and the European Union. The EU's foreign policy chief has warned that ties are reaching a "watershed moment" over Turkish oil prospecting in waters claimed by Greece and Cyprus, saying that sanctions could be imposed next month.
The frigate was operating in the Mediterranean as part of the EU's Irini mission, which aims to stop arms reaching Libya, where Turkey backs the internationally recognised government in its fight against Russian and Egyptian backed rebels in the country's east.
Turkey said the vessel was carrying humanitarian aid and the soldiers had found nothing suspicious. A German Defence Ministry spokesman also said they had not found anything suspicious by the time they were ordered off the ship.
Turkey said the Hamburg violated international law by not waiting for permission to board. Germany said they had sought permission, and that, after four hours had passed with no reply, it was standard practice to believe there was implicit permission.
Turkey released footage showing armed men in military uniform marshalling sailors with their hands on their heads on the bridge of what it said was the Roseline A.
Turkish Foreign Ministry Spokesman Hami Aksoy said: "The captain showed cooperation and shared information about the ship's freight and its course. Despite this, at 17:45, armed forces from the Irini Operation boarded the ship and carried out a 'monitoring' that lasted long hours."
"We protest this act, which was carried out by force and without authorisaton (and) retain the right to seek compensation," he said.
Confirming a report in the news magazine Der Spiegel, the German spokesman said the Hamburg had intercepted the Rosaline A about 200 km (125 miles) north of the Libyan city of Benghazi on Sunday night.
"All procedures were followed correctly," a German Foreign Ministry spokeswoman said, adding that the sailors had withdrawn as soon as Turkey objected.
The 16,000-tonne container ship left the Turkish port of Gemlik near Bursa last week, and was last seen off Athens, heading southwest towards Libya, according to Refinitiv Eikon data.