Russian President Vladimir Putin said Tuesday that Ukraine's ousted president Viktor Yanukovych remained its only legitimate leader despite fleeing to Russia but conceded he lacked any political future.

"I think he has no political future -- I told him that. As for playing a role in his fate, we did that purely from humanitarian reasons," Putin said of Yanukovych, who took refuge in southern Russia after crossing from Ukraine in a way that has not been made public.

Putin said Russia viewed Yanukovych as legally Ukraine's president.

"The legitimate president, purely legally, is undoubtedly Yanukovych," Putin said.

Crucially, Putin said this meant Russian intervention in Ukraine would be justified because Moscow had received a request for protection of its citizens from Yanukovych.

"We have a direct request from the acting and legitimate -- as I have already said -- president Viktor Yanukovych about using armed forces to protect the lives, health and freedom of Ukrainian citizens," Putin said.

Russia's ambassador to the United Nations, Vitaly Churkin showed the letter dated March 1 at Security Council talks on Monday.

Putin spoke scornfully of his erstwhile ally Yanukovych, who drew his supporters from the Russian-speaking part of Ukraine and in November agreed to scrap a deal with the European Union under pressure from Russia.

"Do you sympathise with him?" one journalist asked.

"No, I have completely different feelings," Putin said.

But he said he believed Yanukovych had risked being killed by protesters when he fled Kiev late last month.

"Death is the easiest (way) to get rid of a legitimate president. That would have happened. I think they would have just killed him," Putin said.

Yanukovych said he fled Ukraine after he and his family received death threats and his car was shot at as he drove out of Kiev.

Putin said that Yanukovych had essentially handed over his powers on February 21 by signing agreements with the opposition leaders last month, counter-signed by international mediators.

He "practically gave up all his powers anyway. And I think he -- and I told him this -- had no chances of being re-elected," Putin said.

He said he understood peaceful protesters against Yanukovych: "Of course people wanted changes."

Asked about a rumour reported in Ukrainian media on Tuesday that Yanukovych had died from a heart attack, Putin denied it.

"After he came to Russia, I saw him once, that was literally two days ago. He was alive and healthy. He will catch a cold yet -- at the funerals of those who spread that information," Putin added with characteristic black humour.