Sean Penn, who is on the ground in Ukraine filming a documentary about Russia's invasion, has declared President Vladimir Putin has made a "brutal mistake".
The actor penned a statement for the Washington Post praising Ukrainian President Volodymr Zelensky and his country people as "historic symbols of courage".
"Already a brutal mistake of lives taken and hearts broken, and if he doesn't relent, I believe Mr Putin will have made a most horrible mistake for all of humankind," he said.
"President Zelensky and the Ukrainian people have risen as historic symbols of courage and principle."
The Mystic River star encouraged the United States to support Ukraine amid the invasion, adding: "Ukraine is the tip of the spear for the democratic embrace of dreams. If we allow it to fight alone, our soul as America is lost."
Vice Studios confirmed Penn was filming a documentary on the conflict last week.
The Office of the President of Ukraine shared a translated statement on Facebook last week acknowledging their gratitude for Penn's interest in documenting the situation as it unfolds.
"The director specially came to Kyiv to record all the events that are currently happening in Ukraine and to tell the world the truth about Russia's invasion of our country," it said.
"Sean Penn is among those who support Ukraine in Ukraine today. Our country is grateful to him for such a show of courage and honesty."