Europe needs to stop being naive when it comes to defending its interests and building its own military capacity, French President Emmanuel Macron said on Tuesday after Greece sealed a deal for French frigates worth about 3 billion euros ($3.51 billion).

France was plunged into an unprecedented diplomatic crisis with the United States, Australia and Britain earlier this month over a trilateral nuclear security deal which sank a multi-billion dollar French-designed submarine contract with Canberra.

"The Europeans must stop being naive. When we are under pressure from powers, which at times harden (their stance), we need to react and show that we have the power and capacity to defend ourselves. Not escalating things, but protecting ourselves," Macron told a news conference with Greek Prime Minister Kyriakos Mitsotakis.

"This isn't an alternative to the United States alliance. It's not a substitution, but to take responsibility of the European pillar within NATO and draw the conclusions that we are asked to take care of our own protection."

Under Tuesday's agreement, Athens agreed to buy three frigates with an option to buy a fourth for about 3 billion euros, a Greek government source told Reuters.

The accord, part of a broader strategic military and defense cooperation pact, comes after Athens had already ordered some 24 Dassault-made Rafale fighter jets this year, making it the first European Union country to buy the fighter jet.