Ukraine President Volodymyr Zelenskiy on Wednesday defended his decision to agree to a hostage-taker's demand that he recommend a Joaquin Phoenix movie on his social media to end a 12-hour stand-off.

Zelensky described how he negotiated personally on Tuesday evening with a gunman who was holding 13 hostages on a bus in the western city of Lutsk.

The president had been given the option of launching an assault on the bus, but declined to do so, not wanting to risk the hostages' safety.

"We have a result; everyone is alive. We are not fighting for [approval] ratings, we are fighting for life," Zelenskiy said in a statement about his decision, after receiving some criticism for giving in to the gunman's demands.

The Security Service of Ukraine (SBU) stated that all 13 hostages were released unharmed after a police stand-off with Maksym Kryvosh, who had threatened to blow up the bus and detonate a second explosive device elsewhere in the city. 

The tense hostage situation ended after the president agreed to post a recommendation of a 2005 U.S. documentary narrated by actor Joaquin Phoenix called Earthlings, which condemns humans' mistreatment of animals.

Zelenskiy said he agreed with Kryvosh, 44, that "he would release three people, and after that I would record a video."

The armed man complied and freed three hostages: a man, a pregnant woman and a child.

In response, the president posted a video message on Facebook and Kryvosh agreed to release all his captives half an hour later, Zelenskiy said. The president later deleted the short video from his account.

The gunman is currently being held in a temporary detention center, police reported. Ukraine's interior minister, Arsen Avakov, said the gunman held "extremist views" and the SBU has classed the incident as an "act of terror" and "hostage-taking," for which Kryvosh could face up to 15 years in prison. 

Avakov also stated that an accomplice of Kryvosh was detained in the eastern city of Kharkiv.

"The film this man is talking about is good. And you don't have to ... create such a nightmare for people all over the country. The film can be watched without it," Avakov added.