The European Union backed member Greece’s plan to double the boundaries of its territorial waters from six to 12 kilometers and snapped that Turkey has no call to try to stop it, heightening the tension.

EU spokesperson Maja Kocijancic told Euractiv that the EU “remains committed to good neighborly relations and respect of international agreements,” a statement the news website said amounted to a rebuke to Turkey over its opposition to plans to extend Greece’s territorial waters.

Former Greek Foreign Minister Nikos Kotzias, in his last act before quitting, said the borders would be extended but only in the Ionian Sea off western Greece and not yet in the Aegean between Greece and Turkey.

The prospect has irked Turkey which doesn’t recognize the law of the sea and has sent warships past Greek islands while President Recep Tayyip Erdogan said he covets the return of islands ceded to Greece in the 1923 Treaty of Lausanne he also doesn’t accept.

Turkish Foreign Minister Spokesperson Hami Aksoy said Turkey wouldn’t allow Greece to extend its sea borders and said it would be a cause of war if it happened.

Prime Minister and Radical Left SYRIZA leader Alexis Tsipras, who took over as Foreign Minister, said it would require legislative approval and not just a decree.

Kocijancic’s mention of “respect for international agreements” refers to the United Nations Convention on the Sea which Greece has signed but Turkey hasn’t. The convention allows a 12-kilometer (7.45-mile) distance from the coastline as the maximum extent for a state’s territorial waters. Some Greek islands are closer to Turkey’s coast than that distance.