The surprise intervention from Russia’s Oleg Anisimov at the closed-door meeting followed an electrifying live statement from his Ukranian counterpart, Svitlana Krakovska, who spoke passionately about her country's plight.

"Let me present an apology on behalf of all Russians not able to prevent this conflict," Anisimov said at the closing plenary of the virtual, 195-nation forum, according to three sources who heard him speak.

Delegates and observers to the sometimes fraught meetings, which had been scheduled to end Friday, were stunned by the back-to-back statements Sunday, according to half-a-dozen participants.

Those who see what is happening, he added speaking in Russian, "fail to find any justification for the attack on Ukraine".

The Intergovernmental Panel on Climate Change (IPCC) provided simultaneous translation of his comments into English. AFP did not have access to the original statement in Russian.

Ukraine’s Krakovska, who has tried to continue working despite the assault on her country, addressed the conference on Sunday morning.

"We will not surrender in Ukraine, and we hope the world will not surrender in building a climate resilient future," she said in English, according to multiple sources.

"Human-induced climate change and the war on Ukraine have the same roots -- fossil fuels -- and our dependence on them," she added.

"Everyone 'in the room' was really moved," said one source, referring to online chats and informal conversations.

The statement by Anisimov -- who expressed "huge admiration" for the Ukranian delegation -- came as a particular surprise.

"He knows that there is a risk for him, it was a very sincere message," said another participant.

When asked by AFP to comment, Anisimov said that his statements "expressed my personal opinion and attitude," and should not be taken as an "official statement of the Russian delegation".

Anisimov is a veteran of the IPCC process, first participating as a scientist and expert on the Arctic region. He contributed to earlier reports as a lead author.

The two-week IPCC meeting, overshadowed by Russia's invasion of Ukraine, was tasked with distilling a 3,500-page report on climate impacts and adaptation into a crucial 40-page "Summary for Policymakers," to be made public on Monday.

Krakovska expressed her sadness that after years of meticulous work by scientists around the world, the IPCC's findings would now have to "compete for media space with war".