Updated

A leading contender in Ukraine's upcoming presidential election says some pro-Russian insurgents in the east of the country only understand "the language of force."

Billionaire chocolate magnate Petro Poroshenko said Wednesday, on a trip to Berlin where he was to meet with Chancellor Angela Merkel, that restoring law and order in the east is a priority and he wants to see economic conditions in the heavily Russian-speaking region improved.

But he says there must be "zero tolerance" for armed separatists who have seized several towns and engaged fought with government forces.

Poroshenko, a front-runner in the May 25 election, says he would be ready to negotiate further decentralization of power in Ukraine and hold a referendum, provided it is free and fair.