Diplomatic efforts by French President Emmanuel Macron in response to the war in Ukraine were a failure and "deeply harmful" for Kyiv, Anders Fogh Rasmussen, the former NATO secretary-general, said in an interview published on Friday.

"It was not a success," Rasmussen, a former Danish prime minister who was one of the world's most-senior diplomats until he left the transatlantic defense alliance in 2014, told French magazine Le Point.

His comments come after criticism, especially in eastern Europe, about how Macron kept an open line with Russian President Vladimir Putin with direct phone calls even after the invasion of Ukraine and has warned against 'humiliating' Russia.

"Macron astonished us at the beginning of the crisis with his, to say the least, unique and critical statement that Putin should not be humiliated and offered an exit ramp. Such statements were disastrous and deeply harmful," he added.

The French presidential office did not immediately reply to a request for comment.

However, Rasmussen's broadside comes at a time Macron is recalibrating his message and striking a firmer tone against Moscow.

In a speech delivered at the United Nations General Assembly, the French leader reiterated his belief that world leaders needed to keep up the dialogue with Russia, adding that he would again talk to Putin in the coming days to address the safety situation linked to nuclear reactors situated in Ukraine's war zones.

But according to observers, Macron's UN speech, in which he accused Russia of a modern-day imperialism and urged developing nations to side against Moscow, marked a shift in tone.

Macron stated that peace talks can only work if Ukraine's "sovereignty is respected, its territory liberated and its security protected."

"Russia must now understand that it cannot impose its will by military means," said Macron.

Rasmussen was unconvinced.

"He has weakened international cohesion, and I think he is now regretting this and trying to regain the initiative," he said in the interview.