(CNN) -- Four explosions that rocked an eastern Ukrainian city on Friday have prompted a terror probe, the Ukrainian News agency reported.
A regional prosecutor's office has started a "terrorist case" into the blasts in Dnepropetrovsk. The explosions went off in the course of 70 minutes and injured at least 27.
Ukraine President Viktor Yanukovych said a special investigation team will be set up to probe the explosions, the news agency reported.
"This is another challenge to the entire country," the president told reporters in the country's Crimea region. "We are thinking how to respond it properly. I am sorry this happened."
The first blast went off in a trash can at a tram stop, injuring 13 people, the country's Emergencies Ministry said. The second followed 40 minutes later near a movie theater. That injured 11, nine of them children. Three were injured in the third blast, and no one was hurt in the fourth, the ministry said.
Of those hurt, 24 have been hospitalized, it said.
Interior Minister Vitaliy Zakharchenko and other government officials traveled to the city after the blasts occurred.
The city, with just over 1 million in population, is one of the largest in the country, the CIA World Factbook says.
The incident raises concerns about security in Ukraine ahead of the European soccer championships, starting in June in both Poland and Ukraine.
But the Union of European Football Associations, the entity overseeing the games, said the incident doesn't change its "confidence in the security measures that have been developed by the authorities."
The closest venue to Dnepropetrovsk will be in a Ukrainian city to the east, Donetsk.