ANKARA

Turkey's top diplomat on Tuesday accused a German frigate of violating international law when it stopped and searched a commercial vessel without the consent of its flag-state or its captain.

"It is a violation of international law to embark on trade ships like pirates," Foreign Minister Mevlut Cavusoglu told reporters in the capital Ankara.

Cavusoglu underlined that both Turkey's Foreign Ministry and its Embassy in Rome, where the headquarters of Operation Irini under which the German frigate Hamburg was operating is located, had made the necessary warnings that against the boarding action that it could not be lawfully carried out without the permission of the flag state.

He added that they negotiated on the issue with the UN, NATO and the International Maritime Organization.

"We'll follow the political and judicial process of the issue. We won't let go of this incident," he said.

Cavusoglu went on to say that Ankara would do whatever is necessary: "Our president's instructions are in this direction as well. We cannot leave anything done to us unanswered."

"We will not only respond on the field, but follow the judicial and politic processes, as well," he added.

Under Operation Irini, a German frigate on Sunday illegally stopped and searched a private Turkish-flagged ship carrying materials humanitarian aid to Libya, drawing condemnation from Turkish leaders.

Turkey has long stated that the arms embargo on the war-torn North African country was being enforced in a manner biased to warlord Khalifa Haftar.

The Turkish ship was only carrying such materials as food and paint to Libya's port of Misrata, and did not violate the UN arms embargo, said the Foreign Ministry.

The operation officially announced that their search had turned up no illegal materials.

'Freedom of navigation should be respected'

Replying to claims of Peter Stano, the EU's spokesperson for external relations, the Turkish Foreign Ministry said on Twitter: "@EUNAVFOR_MED [Operation Irini] boarded Roseline-A unlawfully and forcefully w/out the flag state consent & ignored the written and oral messages sent by Turkish authorities before the unlawful boarding."

Stating that Operation Irini changed its position after Ankara protested against the "unlawful" search of its vessel, despite claiming earlier that they did not need the consent of the flag state.

"When Turkey protested this unlawful boarding & reserved its rights for compensation, IRINI changed its position & realized that they could not board the ship without the flag-state consent," it said, adding: "International law and freedom of navigation should be respected, at all times."

* Writing by Dilan Pamuk in Ankara