A series of terrorist attacks in the city of Dnipropetrovsk have hit Ukraine just weeks before it is due to co-host the Euro 2012 football championship.
Four bomb blasts rocked the city, which is 250 miles southeast of Kiev, on Friday and injured 27 people; some witnesses reported that devices had been placed in nearby rubbish bins.
''No-one has been detained yet,'' a police spokesman said.
Dnipropetrovsk is the birthplace of Yulia Tymoshenko, the former Ukrainian prime minister who was jailed on charges of abuse of power and has recently protested via hunger strike over the conditions in which she is being kept.
President Viktor Yanukovych, who has faced criticism over his handling of the case, said: "We understand that this is yet another challenge for us, for the entire nation."
England's first group stage match against France on June 11 is due to be held at the Donbass Arena, 150 miles away in Donetsk.