Media playback is unsupported on your device Media caption The BBC's David Stern: "The president said this is yet another challenge to the country"

Four explosions have rocked the city of Dnipropetrovsk in eastern Ukraine, injuring at least 27 people.

The first two blasts happened at a busy central tram stop and near a local cinema, officials said.

All the devices were apparently planted in rubbish bins.

No suspects have been identified yet and such an attack is unusual for Dnipropetrovsk. The industrial city is the birthplace of jailed opposition leader Yulia Tymoshenko.

"This is yet another challenge for us, for the entire nation," President Viktor Yanukovych said.

Nine children were among the 27 injured in the blasts, which all happened within an hour.

Tensions are running high over the alleged mistreatment of Ms Tymoshenko. She was a leader of the pro-Western Orange Revolution in 2004 but her chief rival, Mr Yanukovych, won an election in 2010 and forged closer ties with Russia.

Ukraine is also preparing to co-host the Euro 2012 football tournament this summer.

Dnipropetrovsk is not a host city in the June-July tournament, but is on the route of the trophy tour that will take place in late May.

Interior Minister Vitaly Zakharchenko flew to the city with the deputy heads of the Prosecutor General's Office and the national security service.

The prosecutors' spokesman said the explosions were being treated as a terrorist attack.