Emmanuel Macron has spoken up for Europe’s ability to defend itself, saying a mutual assistance clause, enshrined in the EU treaty, was unambiguous and “not just words”.

The French president said the pact had already been proved in action when several member states sent military aid to Cyprus after a drone attack against a British airbase on the island on 28 February.

“On article 42, paragraph 7, it’s not just words,” the French leader said. “We know that for us, it ⁠is clear ‌and ‌there is no room ‌for interpretation or ‌ambiguity.”

Macron, in Greece to renew a bilateral strategic defence agreement, described the clause as “stronger” than article 5, Nato’s collective defence clause, as he reiterated his long-held belief that Europe was better off boosting its own security than relying on an increasingly erratic US under Donald Trump. “I really believe this US approach will last,” he said.

A day earlier, EU leaders, attending an informal council in Cyprus, said plans were being finessed on how the obscure clause would work in practice. Speaking on Friday, the European Council president, António Costa, said: “We are designing the handbook [on] how to use this mutual assistance clause.”

Macron questioned the efficacy of the Nato article when asked about the military alliance and its founding principle under which members come to one another’s aid if they are attacked.

“There is now a doubt on article 5, not put on the table by the Europeans but by the US president,” he told the audience during a discussion held with the Greek prime minister in the capital’s picturesque Roman-era agora. “It is clearly a de facto weakening of the Nato alliance … I am a strong believer in the European pillar of Nato and my view is that we should strengthen this European pillar.”

His Greek counterpart, Kyriakos Mitsotakis, appeared to agree, calling the decision to rush fighter jets and naval support to Cyprus “a gamechanger” for the bloc.

Amid fears of the union’s easternmost member coming under sustained retaliatory attack in the first days of the US-Israeli war against Iran, France, Greece, Spain, Italy, the Netherlands and Portugal scrambled to send assistance to the island.

“What we did in Cyprus was a gamechanger,” said Mitsotakis, insisting that the time had come for the little-known defence pact to be taken seriously.

“We have a mutual assistance clause in our treaties and this is our European responsibility. We never spoke about it because we thought that Nato would always do the job … we need to take this article much more seriously; we need to look at the Cypriot lesson, think about what could happen in another case, have exercises in terms of what it would mean if we were again to offer support to a European country under threat.”

Doing so would be tantamount to a “political statement” that the EU did not only rely on Nato, and would be “also good for Nato”, he added.

Infuriated by Nato’s failure to support the strikes against Iran, the US president has stepped up criticism of the transatlantic alliance, further raising concerns that support for article 5 from Washington can no longer be guaranteed.

Macron, who is making his third official visit to Greece before he leaves office next year, said the strong alliance between the two nations should serve as a model for the rest of the EU.

On Saturday an unprecedented nine accords were signed between the countries, foreseeing increased cooperation in areas including scientific research and nuclear technology. Macron vowed that France would stand by Greece if it ever came under attack from its neighbour and long-time regional rival, Turkey.

In 2017, Macron, then newly elected, had used the dramatic setting of the ancient Pnyx beneath the Athens Acropolis to give a rousing policy speech on the future of Europe and the virtues of democracy.

The tone, nine years later, could not have been more different. At a time of such geopolitical uncertainty, Europe, he said, had to “wake up” and claim its place as a geopolitical power as it faced opponents it had not faced before.

“We should not underestimate that this is a unique moment where a US president, a Russian president and a Chinese president are dead against the Europeans,” he told the crowd. It now remained for a continent that had managed to end centuries of civil war – and deliver on prosperity – to “write the next chapter and become a geopolitical power”.