The move raised eyebrows in Ankara which warned similar actions to the east could raise the prospect of armed conflict between the two NATO allies. Greek Prime Minister Kyriakos Mitsotakis made the announcement during a debate on whether to approve an accord on maritime boundaries between Greece and Italy.
He told MPs: “Greece will extend its western territorial waters to twelve nautical miles from six.”
The planned measure does not affect the Aegean region off Greece’s eastern and southern coasts.
But Turkey has warned a similar move by Athens to the east of Greece would be a “casus belli” – or cause for war.
The two countries are in dispute over the extent of their continental shelves and their maritime boundaries and the row escalated after Turkey sent a survey vessel in disputed eastern Mediterranean waters this month, a move Athens branded illegal.
Donald Trump spoke with Mr Mitsotakis yesterday to discuss the latest developments in the eastern Mediterranean.
Athens said Mr Mitsotakis told Mr Trump Greece was ready to help de-escalate tensions as long as Turkey ended its provocations.
The White House said Mr Trump had expressed his concern about the increased tensions between Greece and Turkey and told Mr Mitsotakis the two NATO allies “must commit to dialogue, which is the only path to resolving their differences”.
Relations between Athens and Ankara have plummeted to their lowest point in more than 20 years.
READ MORE:EU on brink: Germany warns ‘any spark’ in Turkey-Greece a threat
Simmering tensions threatened to boil over earlier this month when a Greek frigate collided with a Turkish ship.
A Turkish vessel, escorted by warships, then began seismic research south of the island of Kastellorizo in what Greece considers its continental shelf.
Warships from both sides have been shadowing each other in the area for several weeks and both have military exercises in recent days.
And a fresh row erupted just yesterday when Athens accused Turkey of trying to intervene when the Greek coast guard launched a mission to rescue dozens of migrants from a sinking boat off the island of Halki.
Turkish President Recep Tayyip Erdoğan said: “Turkey will take what is its right in the Mediterranean, in the Aegean and in the Black Sea.
“We invite our interlocutors to get their act together and to avoid mistakes that will lead to their ruin.”
Earlier this week, Mr Erdoğan accused Greece of “sowing chaos” in the Mediterranean.
Both Greece and Turkey have said they are ready for dialogue but continue to blame each other.
Mr Mitsotakis said:”Our position is crystal clear and can be summarized in six words: When provocations stop, talks can start.”
German Foreign Minister Heiko Maas has visited both Athens and Ankara aiming to calm tensions.
Mr Maas said: “The current situation in the eastern Mediterranean is equivalent to playing with fire – every little spark can lead to catastrophe.
“No one wants to settle this conflict militarily, which would be absolute madness.”