The French military on August 13 conducted training exercises with Greek forces off the southern island of Crete, defence sources briefed Reuters, as tension persisted with Turkey over disputed waters overlaying potential gas and oil deposits in the eastern Mediterranean.

The exercise was the first manifestation of French President Emmanuel Macron’s commitment to temporarily reinforce his country’s presence in the eastern Mediterranean as France ups pressure on Turkey to halt oil and gas exploration in the contested area. Greece has placed its military forces on high alert, recalling its naval and air force officers from holiday.

Greek Prime Minister Kyriakos Mitsotakis discussed the situation in the region with Macron by telephone on August 12. “Emmanuel Macron is a true friend of Greece and a fervent defender of European values and international law,” Mitsotakis tweeted, in French, after the call with Macron.

Macron is also unhappy with Turkey’s aggressive role backing one side in the Libyan Civil War. The outcome of the conflict could potentially leave Turkish and Russian military bases staring across the Mediterranean at southern Europe the way things stand, analysts say.

The French armed forces ministry said it was sending two Rafale fighter jets and the naval frigate ‘Lafayette’ to the eastern Mediterranean. The quoted sources said they arrived in Crete earlier on August 13 and carried out joint manoeuvres with Greek forces.

A Turkish seismic vessel, the Oruc Reis, escorted by gunboats, has been sailing between Crete and Cyprus since August 10. It was dispatched by Ankara days after Greece signed a maritime deal with Egypt designating an exclusive economic zone between the two nations. Turkey says it plans to open up some of the area for potential hydrocarbon exploration.

There is speculation that Turkish President Recep Tayyip Erdogan is attempting to divert attention from his country’s worsening economic woes by fanning the flames of territorial disputes with Greee and Cyprus and issuing bold comments about Turkey’s military activities in Libya, Syria and Iraq.

“Our country does not threaten [anyone], but nor can it be blackmailed,” Mitsotakis told Greece on the evening of August 12. “Let it be known to all: the risk of an accident lies in wait when so many military forces gather in a limited area.”

Turkey’s prospecting should “cease in order to allow a peaceful dialogue between the neighbouring Nato members”, Macron tweeted in a strong statement, describing the situation as “preoccupying”.

Last month Macron called for EU sanctions against Turkey. He hit out at Ankara’s “violations” of Greek and Cypriot sovereignty over their territorial waters.

The EU’s foreign policy chief, Josep Borrell, referred to the mobilisation of naval forces as “extremely worrying”. The bloc was racing to arrange an emergency foreign affairs council for August 14.

“On a scale of one to 10, I would say bilateral tensions are at a level of seven to eight,” Constantinos Filis, a professor of international relations, was quoted as saying by the Guardian. “This could easily erupt into the most serious crisis between Greece and Turkey in almost 25 years. Room for human error is very real if Turkey continues on this path of revisionist brinkmanship.”

It was, he added, “simply illegal” to conduct seismic research in waters that had neither been defined by an agreement nor the verdict of an international court.

Turkish defence minister, Hulusi Akar, pledged that Ankara would protect “its rights, ties and interests” in coastal areas. “Despite all this, we want to believe that common sense will prevail … it should be known that our seas are our blue homeland. Every drop is valuable,” Akar told Reuters.

Athens maintains that its islands—no matter how small—have their own continental shelves, but Ankara argues that if that principle was upheld the Aegean sea would effectively be turned into a Greek lake. Turkey, almost entirely dependent on energy imports, says that is utterly unacceptable to a country that sees itself as a regional power and has no intention of being left out of any big energy payday.