Ukrainian authorities say 10 civilians have been killed and 13 wounded by a missile that hit a bus in eastern Ukraine.
The incident occurred on January 13 near a Ukrainian military checkpoint close to the town of Volnovakha, 35 kilometers southwest of the rebel-held provincial capital of Donetsk.
One report said the bus was carrying civilians from the coastal city of Mariupol.
Ukraine's military said the civilians were killed by Grad rockets, one of which hit a bus.
It said the rockets were fired by pro-Russian separatists forces from a position in the town of Dokuchayevsk.
The separatist, self-proclaimed Donetsk People's Republic denied responsibility, saying the government checkpoint is "beyond our artillery's range."
Separatist leaders in Donetsk said they "do not rule out this could have been an act of provocation by the Ukrainian military to heap blame for what happened" on separatist fighters.
The Donetsk regional administration, which answers to the central government in Kyiv, is saying that at least 10 civilians were killed and 13 were wounded.
A cease-fire agreed in Minsk in September is violated almost daily, and more than 4,700 people have been killed and over 10,000 injured in fighting between government forces and rebels in eastern Ukraine since April.
Fighting has intensified recently, jeopardizing hopes for a peace deal.
Separatists and Ukrainian forces have traded accusations of violating a truce that had been in place since early December. Meanwhile, Moscow has accused Kyiv of trying to seek a military solution to the conflict.
On January 12, the foreign ministers of Ukraine, Russia, Germany, and France, meeting in Berlin, failed to make sufficient progress to warrant holding a four-nation summit this week as part of efforts to end the conflict.
German Foreign Minister Frank-Walter Steinmeier made the announcement after four hours of talks in Berlin with Russia's Sergei Lavrov, France's Laurent Fabius and Pavlo Klimkin of Ukraine.
Steinmeier said the meeting had been a "very open exchange" but did not produce the results needed for the countries to go ahead with a meeting of leaders this week in Kazakhstan as originally proposed last month by Ukrainian President Petro Poroshenko.
Steinmeier said representatives from their ministries would meet in the coming days to see if they could bridge differences.
"If there is progress made at that level in the coming days, then we are prepared to meet again next week and resume this discussion we began today," he told reporters.
In a joint statement, the four ministers also called on the contact group of Ukraine, Russia, and the Organization for Security and Cooperation in Europe (OSCE) to meet to try and make progress on implementing a much-violated Ukraine peace deal drawn up in September in Minsk, including creating the "relevant conditions for an effective cease-fire."
If this is done, it would "pave the way to the preparation of a successful summit meeting" in Kazakhstan, they wrote.