The EU has asked NATO for help with its naval mission enforcing an arms embargo on Libya, officials said Friday, after Turkey blocked it from inspecting a suspect vessel.
Turkey's resumed search for oil and gas in the eastern Mediterranean is "extremely worrying", the EU said Sunday after Greece and Egypt set up an exclusive economic zone in the region.
"Latest naval mobilisations in eastern Mediterranean... will lead to a greater antagonism and distrust," foreign policy chief Josep Borrell said, calling the development "extremely worrying".
"Maritime boundaries must be defined through dialogue and negotiations, not through unilateral actions and mobilisation of naval forces," Borrell said in a statement.
The discovery of vast gas reserves in the region in recent years has sparked a prospecting scramble by Greece, Turkey and Egypt as well as Cyprus and Israel.
Turkish President Recep Tayyip Erdogan announced Ankara's resumption of operations on Friday, accusing arch-foe Greece of failing to keep its promises.
His comments came a day after Athens and Cairo signed an agreement to set up an exclusive economic zone in the region.
On Sunday, Borrell said "disputes must be solved in accordance with international law," adding that Brussels was "committed to help solving such disputes and disagreements in this area of vital security interest".
He added: "The present course of action will not serve the interests either of the European Union, or of Turkey. We have to work together for the security in the Mediterranean."
The deal between Greece and Egypt aimed to establish maritime boundaries between the two countries and appeared to be a direct response to a similar accord reached last November between Turkey and the UN-recognised government in Libya.
The agreement considerably enlarged Turkey's maritime territory and drew accusations from several countries, led by Greece, that Ankara was trying to assert its dominance in the region.