Police sealed off the center of Lutsk, a city 400 kilometers (250 miles) west of Kyiv. The assailant is armed and carrying explosives, they said in a Facebook statement.

Police officers are trying to get in touch with the man. He called the police himself at 9:25 a.m. (0725 GMT) after taking control of the bus and introduced himself as Maksim Plokhoy, Deputy Interior Minister Anton Gerashchenko said on Facebook.

Gerashchenko said there is a book online, signed by Maksim Plokhoy and titled "Philosophy of a criminal,” describing a man's experience in prison.

"For 15 years they've been correcting me, but I haven't been corrected, on the contrary - I've become even more who I am,” one extract from the book said, according to Gerashchenko.

Interior Minister Arsen Avakov is flying to Lutsk.

Ukrainian media reported that gunshots could be heard at the scene. It was not immediately clear whether anyone has been injured.

Ukrainian President Volodymyr Zelenskiy said that the assailant took control of the bus at 9:25 a.m. local time (0725 GMT).

"Gunshots have been heard, the bus is damaged,” Zelenskiy said in a Facebook statement, adding that measures are being taken to resolve the situation without casualties.

"I keep the course of events under personal control," Zelenskiy said.

Photos published on social media showed a small bus parked in the middle of an empty street. Two windows of the bus were smashed and other windows were covered with curtains.

Police said they had blocked off the city centre.

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