KYIV, Ukraine (AP) — An armed man seized a bus and took some 20 people hostage in northwestern Ukraine on Tuesday, Ukrainian police said.

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. They have already identified him and said that he expressed frustration with “Ukraine's system” on his social media pages.

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