Greece ratified an accord on maritime boundaries with Egypt on Thursday, hours after Turkey extended the operation of a seismic survey vessel in the Eastern Mediterranean and said it will hold firing exercises in the region next month.
NATO allies Greece and Turkey are at odds over the rights to potential hydrocarbon resources in the area, based on conflicting claims over the extent of their continental shelves.
Tensions escalated this month after Ankara dispatched the Oruc Reis seismic survey vessel in a disputed area following the pact between Athens and Cairo.
Turkey has said the pact infringes its own continental shelf. The agreement also overlaps with maritime zones Turkey agreed with Libya last year, decried as illegal by Greece.
The Aug 6 maritime deal has already been ratified by Egypt's parliament and was approved by a majority of Greek lawmakers on Thursday evening.
Earlier, the Turkish navy issued the latest advisory, known as a Navtex, saying it will hold the shooting exercises in the eastern Mediterranean off the coast of Iskenderun, northeast of Cyprus on Sept 1-2. It also extended the seismic work of the Oruc Reis vessel southwest of Cyprus, until Sept 1.
Greece says the Turkish advisories are illegal.
Maritime zones give a state rights over natural resources. Largely unexplored, the east Mediterranean is thought to be rich in natural gas.
As the dispute widened, France said on Wednesday it was joining military exercises with Italy, Greece and Cyprus in the eastern Mediterranean.
Turkish Foreign Ministry spokesman Hami Aksoy said the deployment of French military aircraft in Cyprus violated treaties regarding the control and administration of the island after independence from Britain in 1960.
Aksoy said that France's stance was dangerously encouraging Greece and Cyprus to further escalate tensions in the region.
Cyprus was divided in 1974 following a Turkish invasion triggered by a Greek-inspired coup. Turkey recognises the Turkish-populated north of Cyprus as a separate state, which is not recognised by other countries.
Greece said on Wednesday it plans to extend its territorial waters in the Ionian Sea to 12 nautical miles from its coast, from six nautical miles, after the ratification of a maritime deal with Italy.
To the east of Greece, Turkey has warned that a similar move by Athens in waters east of Greece would be a cause for war.