Markus Schreiber, Associated Press
Ukrainian presidential candidate and businessman Petro Poroshenko briefs the media after a meeting with Germany's Christian Union's faction law makers in Berlin, Germany, Wednesday, May 7, 2014. The Ukrainian government is planing a presidential election on May 25, 2014.

BERLIN — A leading contender in Ukraine's upcoming presidential election vowed Wednesday to crack down on pro-Russia insurgents in eastern Ukraine, claiming that some of them only understand "the language of force."

Billionaire chocolate magnate Petro Poroshenko told reporters ahead of a meeting with German Chancellor Angela Merkel in Berlin that while he was willing to hear and ease the grievances of Ukrainians in the country's heavily Russian-speaking east, restoring law and order was a key priority.

"We should speak to the people living in the east — speak and understand them," said Poroshenko. "For those people who are terrorists, we should find out the right language they understand — and that would be the language of force."

"We should have zero tolerance for terrorists," he added.

Poroshenko slammed plans for a referendum this Sunday by pro-Russia insurgents pushing for more autonomy or even independence in eastern Ukraine, saying there was "no legal basis" for such a vote.

But the front-runner in Ukraine's May 25 presidential election said he would be ready to negotiate further decentralization of powers and even hold a referendum on constitutional changes in Ukraine, provided it was free, fair and not "under the machine gun or automatic rifle."

In Moscow, Russian President Vladimir Putin said the Sunday autonomy vote in eastern Ukraine should be postponed.

In addition to Merkel, Poroshenko also met Wednesday with German Foreign Minister Frank-Walter Steinmeier and Andreas Schockenhoff, a lawmaker with Merkel's conservative party.

Schockenhoff, who once was the government's special adviser on Russia, urged Moscow to prevent any further escalation of tensions in Ukraine.

"There are indications that operations in the east and south of Ukraine are directly steered by high-ranking Russian politicians," Schockenhoff said, without elaborating. "That must be stopped."