Four bombs have exploded at short intervals in the Ukrainian city of Dnipropetrovsk in what authorities said amounted to an "act of terrorism", wounding 27 people, including nine children.

President Viktor Yanukovich said Friday's blasts, from bombs planted in rubbish bins, represented "a challenge ... to the whole country."

The authorities were considering how to respond "appropriately", news agencies quoted him as saying.

Al Jazeera's Sue Turton reports from Moscow.