SAN FRANCISCO, United States-President Joe Biden had a private, emotional meeting with the widow and daughter of Alexei Navalny in California on Thursday, as his administration prepared to announce fresh sanctions against Russia over the death of the Kremlin opposition leader.

The visit at hotel in San Francisco came as the White House backed Navalny's mother in her fight to retrieve her son's body, which Russian authorities have refused to release days after he died in an Arctic prison.

Navalny's team says the 47-year-old, President Vladimir Putin's most outspoken critic, was murdered.

Biden could be seen hugging Yulia Navalnaya, Navalny's widow, and leaning forward as he spoke with her and daughter Dasha, a student at Stanford University, in images released by the White House.

The president's own history of loss -- his first wife and his infant daughter were killed in a car crash in 1972, while his son Beau died of cancer in 2015 -- has seen him often referred to as America's Consoler-in-Chief.

After the meeting, he said the two women were emulating Navalny's "incredible courage."

Yulia Navalnaya, who has vowed to continue her late husband's opposition to Putin, is "not giving up," he said.

- 'Polar Wolf' -

Biden also took direct aim at the Russian leader, reaffirming that his administration will unveil sanctions Friday "against Putin, who is responsible for his death."

The US and its allies have imposed a slew of sanctions on Russia since it invaded Ukraine in February 2022.

Navalny, who died on February 16, galvanized mass protests against Putin, winning popularity with a series of investigations into state corruption.

He was poisoned with a Soviet-era nerve agent in 2020, then jailed in 2021 after returning to Russia following a period of treatment in Germany.

He was sentenced to 19 years in prison on extremism charges and sent to IK-3, a harsh penal colony beyond the Arctic Circle known as "Polar Wolf".

Western governments and Russian opposition figures have accused the Kremlin of being responsible for his death, with an outraged Biden previously blaming Putin and his "thugs."

- Mother's plea -

Hundreds of people have been detained in Russia in recent days at events to pay tribute to Navalny.

His mother, Lyudmila Navalnaya, travelled to Russia's Far North the morning after her son's death was announced, hoping to retrieve his body.

For days officials refused her access, prompting her to make a video appeal directly to Putin himself.

On Thursday she said she had been shown Navalny's body in a morgue in Salekhard, the nearest town to the remote prison.

But, in a video released on social media by Navalny's team, she said investigators wanted her son to be buried "secretly, without a chance to say goodbye.

"They are blackmailing me, they put conditions for where, when and how Alexei should be buried. This is illegal," she said.

"They want to take me to the edge of a cemetery to a fresh grave and say: Here is where your son lies. I am against that.

"I want that for those of you for whom Alexei is dear, for everyone for whom his death became a personal tragedy, to have the possibility to say goodbye to him."

She said she recorded the video because investigators were "threatening" her.

"Looking me in the eye, they said that if I do not agree to a secret funeral they'll do something with my son's body... I ask for my son's body to be given to me immediately," she said.

Navalny's mother also said that investigators had told her they knew the cause of death but did not say what it was.

The Kremlin has refused to say when the body will be handed over and has branded Western accusations as "hysterical."

"The Russians need to give her back her son," US National Security Council spokesman John Kirby told reporters Thursday.

Putin has remained silent on the death of his main political opponent.

Navalny's spokesman Kira Yarmysh said a medical report on the death shown to Lyudmila Navalnaya "stated that the cause of death was natural."