At least 27 people have been injured in a series of bomb blasts in the Ukrainian city of Dnipropetrovsk.

Authorities said four bombs were planted in rubbish bins and exploded at short intervals.

The first blast was at a tram stop at 11.50am (9.50am Irish time), injuring five people.

The second explosion happened 30 minutes later next to a cinema, injuring seven people.

"The number of wounded is 27 at the moment," the Interior Ministry said.

The country's president, Viktor Yanukovich, promised an adequate response to the bomb blasts.

"We ... understand that this is another challenge to us, to the whole country," Mr Yanukovich said.

"We will think how to respond accordingly."

Separately, the prosecutor general's office said it had launched a criminal case classifying the blasts as an "act of terrorism".

Dnipropetrovsk, 400km (250 miles) southeast of the capital Kiev, is one of the former Soviet republic's biggest industrial hubs and was a key centre of the nuclear, arms and space industries in Soviet times.