Actor and humanitarian Sean Penn is currently on the ground in Ukraine to document the growing Russian aggression. 

Shocked and angered by the conflict, the 61-year-old actor posted about what he has witnessed on Twitter. In the post, Penn slams Russian President Vladimir Putin and praises the way Ukrainian leader Volodymyr Zelenskyy has responded to the conflict. 

"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," Penn declared in the statement. 

What Penn has seen first-hand, while capturing the impact of the conflict for a documentary, saw him praising the Ukrainian people for their "courage". 

"President Zelenskyy and the Ukrainian people have risen as historic symbols of courage and principle," he wrote. "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." 

The Ukrainian Prime Minister released a statement last week announcing that Penn had arrived in Kyiv to film a documentary. 

"American actor and film director, Oscar winner Sean Penn arrived in Ukraine," a translated statement on the Ukrainian President's official Facebook page stated. 

"The director came to Kyiv specifically to record all the events taking place in Ukraine as a documentary filmmaker and to tell the world the truth about Russia's invasion of our country." 

"Today, Sean Penn is among those who support Ukraine in Ukraine," the statement added. 

Penn first journeyed to Ukraine in November last year to prepare for the documentary. At the time he was photographed visiting the Azov region and talking with the Ukrainian military. 

During his latest visit, Penn was seen meeting with Ukrainian presidential adviser Alexander Rodnyansky. The 44-year-old Ukrainian posted a photo of the two of them to his own Twitter account, recalling his "captivating conversation" with Penn. 

"On a slightly more positive note, today in spite of the political and military situation: Exciting, fun and captivating conversation over dinner with @SeanPenn, who came to #Kyiv at this critical moment. #StandWithUkraine," Rodnyansky tweeted.