Sentsov, a native of Ukraine's Crimea peninsula, was jailed after Russia seized and annexed it in 2014, and has become a symbol of defiance in Ukraine. Russian authorities sentenced him to 20 years for plotting terrorist attacks, in what human rights defenders say was a politically motivated case.

In a letter from jail, Ukrainian filmmaker Oleg Sentsov has vowed “not to give up,” saying he still believes in a good outcome but does not know how much longer he’ll last.

“Thank you to those who still, despite this endless marathon, continue to support me and, most importantly, other Ukrainian political prisoners,” Sentsov wrote in a letter to human rights activist Zoya Svetova, who shared the text, which she said was written on Tuesday, on her Facebook page.

“They’re not giving up and I’m not either. I believe in a good outcome to this story, despite everything. Nothing is forever, not even the bad."

Sentsov, who on Wednesday entered his 136th day of hunger strike, added he hadn’t expected to “last this long."