French President Emmanuel Macron attends a joint press conference with Russian President Vladimir Putin (off frame) in Moscow, early on 8 February 2022.

French President Emmanuel Macron said Thursday that accusing Russia of committing genocide in Ukraine could lead to the war spreading, as he defended his decision not to use the term.

"The word genocide has a meaning" and "needs to be characterised legally, not by politicians," Macron told France Bleu radio during a trip to northern France.

"States that consider that it is genocide have an obligation under international law to intervene. Is that what people want? I don't think so," he explained.

It would mean becoming a "co-belligerent" in the war, he said, potentially drawing Western countries in the NATO military alliance into the conflict.

Forensic technicians exhume the bodies of civilians who Ukrainian officials say were killed during Russia's invasion and then buried in a mass grave in the town of Bucha, outside Kyiv, Ukraine on April, 2022

"Everyone needs to keep a sense of proportion, it's not helping Ukraine... to enter into verbal escalations without drawing all of the conclusions," he added.

Macron revealed he had spoken on Thursday morning to Ukrainian President Volodymyr Zelensky, who had vowed to discuss the use of the term genocide with Macron.

On Wednesday, Zelensky had taken issue with Macron's refusal to use the word genocide and his description of Ukrainians and Russians as "brotherly people".

A resident walks near a building destroyed in the course of the Ukraine-Russia conflict, in the southern port city of Mariupol, Ukraine on 10 April, 2022

"Such things are very painful for us," Zelensky said. Another call between them was scheduled for later Thursday, Macron said.

Zelensky has repeatedly branded Russia's military onslaught a "genocide", a term first coined by the Polish lawyer Raphael Lemkin in the 1940s.

US President Joe Biden and Canadian Prime Minister Justin Trudeau have also used the word, but others such as Macron and German leader Olaf Scholz have steered clear.

UN Secretary-General Antonio Guterres has also declined to refer to genocide.

"Genocide is strictly defined in international law. And for the UN, we rely on the final legal determination by the appropriate judicial bodies," he said Thursday.

The 1948 UN Genocide Convention describes genocide as acts committed "with intent to destroy, in whole or in part, a national, ethnic, racial or religious group."

Cecily Rose, a professor of international public law at Leiden University in the Netherlands, told AFP that the term "should be used by politicians with great care and caution and preferably on the basis of an independent fact-finding body."