Kiev(Ukraine): Four blasts within minutes rocked the centre of the eastern Ukrainian city of Dnipropetrovsk on Friday, injuring at least 27 people in what prosecutors believed was a terrorist attack, officials said.

Emergency Situations Ministry spokeswoman Yulia Yershova said the first blast occurred at a tramway stop in the centre of Dnipropetrovsk, injuring five people. The second injured seven people outside a movie theatre, while the third wounded three near a railway station.

A fourth blast was also heard in the city centre, the Interior Ministry said. It was unclear whether anybody was injured.

Interior Minister Vitaly Zakharchenko and other top law enforcement officials were flying to Dnipropetrovsk. Prosecutors' spokesman Yuri Boichenko said investigators are treating the blasts as a terrorist attack.

In January 2011, two pre-dawn explosions outside an office of a coal mining company and then a shopping center in the eastern city of Makiyivka caused no casualties.

The authorities said they received letters from unknown assailants demanding money in exchange for not causing any more blasts, and they treated those explosions as acts of terrorism.