Sean Penn in Kraków. Actor's NGO pledges to support refugees from Ukraine

At a news conference in Kraków, the actor acclaimed for Oscar-winning roles in "Mystic River" and "Milk", and co-founder of the global crisis relief NGO 'CORE', signed a Memorandum of Understanding (MoU), reaffirming the organization’s commitment to supporting Ukrainian refugees by providing services on the ground in Poland.

Penn, 61, said CORE was very motivated to help. "Any thinking person knows, that the ramifications of this horrifying invasion go far beyond Ukraine, and will come to all of our doorsteps," he said, adding "this is a game-changing moment for mankind."

"Today, I think, is a genuine tipping point. I feel that as Americans, really specific to this moment ... it has been building, but we are now, in my view, falling from a cliff. And on that cliff is the American flag, and we are plummeting, and we are about to lose sight of that flag. We won't be able to see it anymore if we don't help Ukraine with everything we've got."

"You know, I am going to do everything I can do. And, whether that takes me being here, or coming back many times, or just coming back to shake the hands of the people who are doing the work... you know, yes. They can, you know, count on me as I can count on them," Penn added.

The actor also said that "crises on this scale take more than government, and it needs citizen participation". "CORE's history has been and will continue to be, that while our initial teams are populated by many ex-pats, we will soon be a Polish-led and staffed organisation and we intend to stay the course," he said.

The actor called the Ukrainian people "extraordinarily resilient", having risen to "historic levels of courage", before complimenting their President Volodymyr Zelenskiy for stepping up on such a challenging occasion. "I find the people and their president so entirely moving," Penn stressed.

He also commended the Polish people for taking in and welcoming Ukrainian refugees but warned of foreseeable rising levels of xenophobia.

Penn himself had fled Ukraine in early March, arriving in Poland on foot along with two colleagues of his.

Penn, whose directorial credits include "Into the Wild" and "The Crossing Guard," is also known for his political activism and involvement in various humanitarian causes, including relief efforts for victims of Hurricane Katrina, the Haiti earthquake of 2010 and Pakistan floods in 2012.

The Oscar winner is producing the Ukraine documentary for Vice Studios, a U.S.-Canadian digital media and broadcasting company, according to Hollywood trade publication Variety.

On February 24, Russian President Vladimir Putin sent his troops into Ukraine on what he calls a "special military operation" to demilitarise and "denazify" Ukraine. Ukraine and the West say Putin launched an unprovoked war of aggression.