MOSCOW, June 19 -- Russian President Vladimir Putin on Thursday expressed hope that Ukrainian authorities would stop violence and start a national dialogue without delay.

"In particular, Putin voiced serious concerns about the military operation that Kiev is continuing in the southeastern part of Ukraine," the Kremlin press service said, citing telephone conversations between Putin and German Chancellor Angela Merkel and French President Francois Hollande.

The leaders also discussed the failure of talks on settling the Ukrainian debt for Russian gas supplies and its possible consequences for Europe's economy and energy security, it said.

On the same day, Russian Foreign Minister Sergei Lavrov, in a phone conversation with his German counterpart Frank-Walter Steinmeier, called for thorough and unbiased investigations into the May 2 tragedy in Ukrainian city of Odessa as well as the recent killings of Russian journalists in that country.

He added that the investigations must involve the Organization for Security and Cooperation in Europe and the United Nations.

Clashes between pro-government paramilitary groups and anti- government protesters in Odessa killed more than 40 people. On Tuesday, two Russian journalists were killed from mortar injuries reportedly resulted from Ukrainian troops.