KIEV, Ukraine (AP) -- Four blasts within minutes rocked the center of the eastern Ukrainian city of Dnipropetrovsk Friday, injuring at least 27 people, including nine children, 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 center of Dnipropetrovsk, injuring 13 people. The second injured 11 people near a school, including the nine children, while the third wounded three near a railway station.

   A fourth blast was also heard in the city center, Yershova said. It was unclear whether anybody was injured.

   Interior Minister Vitaly Zakharchenko and the deputy heads of the Prosecutor General's Office and the national security service were flying to Dnipropetrovsk. Prosecutors' spokesman Yuri Boichenko said investigators are treating the blasts as a terrorist attack.

   President Viktor Yanukovych called the explosions ``yet another challenge for the whole country,'' and said that Ukraine's best investigators will be working on the case, according to the Interfax news agency. ``We will think of a worthy response.''

   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 then received letters demanding money in exchange for an end to the blasts. The perpetrators were later detained and sentenced to lengthy prison terms.