An armed man carrying explosives has taken around 20 passengers hostage on a bus in the western Ukrainian city of Lutsk, police said Tuesday.
The head of the local police service said shots were heard at the scene but no injuries have been reported so far.
Law enforcement has cordoned off the centre of Lutsk, a city in western Ukraine some 400 kilometres (250 miles) from the capital Kiev, and advised residents not to leave their homes or places of work.
Police said the SBU security services had surrounded the minibus after two shots were fired from it towards law enforcement.
"The attacker threw a grenade from the bus, which, fortunately, did not detonate," a statement said, adding that the attacker was believed to have undergone psychiatric treatment.
Video footage and pictures published by local media showed heavily armed police in Lutsk surrounding a blue and white minivan with several windows shattered and its curtains drawn.
The hostage-taker made contact with the police and identified himself as Maksym Plokhoy, deputy interior minister Anton Gerashchenko said.
The authorities were working to confirm the identity of the attacker, he told AFP.
Gerashchenko said law enforcement was talking with the assailant in the hopes of resolving the crisis "through negotiations".
- 'Anti-system' suspect -
Posts on social media accounts using Plokhoy's name claimed he was armed, including with bombs.
They described him as "anti-system" and made demands of the authorities.
The interior ministry told AFP it believed the accounts were genuine.
President Volodymyr Zelensky said news of the hostage taking was "disturbing".
"Every effort is being made to resolve the situation without casualties," he said on Facebook.
Interior Minister Arsen Avakov was travelling to the region to coordinate a resolution to the crisis, the ministry said.
Ukraine, which has been fighting Russian-backed separatists since 2014, has been struggling with a proliferation of illegal weapons.
The fighting broke out between Kiev forces and Russian-backed separatists in the eastern Donetsk and Lugansk regions after Russia annexed the Crimean peninsula from Ukraine in 2014.
More than 13,000 people have been killed in the fighting so far.
Police in late 2017 stormed a post office in the eastern city of Kharkiv, where an armed man claiming to be strapped with explosives had captured 11 people.
Map locating Lutsk, in Ukraine, where an armed man has taken passengers hostage on a bus.
Posts on social media accounts using Plokhoy's name described him as 'anti-system'