Ukrainian President Petro Poroshenko has called on Russia to support his peace plan "with deeds, not words."
Speaking at the Parliamentary Assembly of the Council of Europe (PACE) in Strasbourg on June 26, Poroshenko said that, despite voicing support for the plan, Moscow was still not doing enough.
Earlier on June 26, U.S. Secretary of State John Kerry said Moscow must prove within "hours" that it is working to support a weeklong cease-fire between Kyiv and pro-Russian separatists in eastern Ukraine.
Kerry said it was critical for Russia to show "in the next hours, literally" that it's moving to help disarm the separatists and encourage them to become part of a "legitimate process."
Kerry said further sanctions would be imposed on Moscow if the situation in Ukraine deteriorated, but that the United States and its allies preferred "not to have to be in sanctions mode."
The Kremlin said Russian President Vladimir Putin and German Chancellor Angela Merkel, in a telephone conversation on June 26, discussed "the need to extend the cease-fire."
The Kremlin said the two leaders also talked about a possible monitoring of the cease-fire, the start of a regular contact group, and the release of hostages.
Putin also pointed to what Moscow says is an increasing stream of refugees from the conflict into Russia.
In Strasbourg, Poroshenko told PACE he had submitted to parliament constitutional amendments that grant more powers to Ukraine's regions.
WATCH: Residents of the eastern Ukrainian city of Slovyansk said shooting continued in the city on June 25 despite a cease-fire announced between Kyiv and pro-Russian separatists. People remaining in the city stocked up on water while others queued to share taxis to leave their embattled hometown.
More than 300 people have been killed in eastern Ukraine in the past weeks as separatist rebels battled government forces.
Sporadic fighting has continued despite a unilateral cease-fire declared by Kyiv on June 20.
Ukrainian news reports said on July 26 that separatists and government troops had exchanged fire at an airfield outside the city Kramatorsk, in the Donetsk region.
One of the leaders of the self-proclaimed "Donetsk People's Republic," Andrei Purgin, said the separatists had agreed to hold further peace talks on June 27 in Donetsk.
With reporting by AP and Interfax