A missile hit a passenger bus in eastern Ukraine Tuesday, killing at least 11 civilians near the separatist-controlled city of Donetsk.
Pro-government officials say the bus was traveling near a military checkpoint when it was struck by a missile from a multiple-rocket launcher.
President Petro Poroshenko said the deaths "chill the heart." He immediately blamed pro-Russian rebels.
A separatist spokesman denied responsibility.
The bus attack is another blow against efforts to implement last September's cease-fire between the Ukrainian government and pro-Russian separatists.
A meeting between French, German, Russian and Ukrainian leaders that was to have been held Thursday in Astana, Kazakhstan has been canceled. Fighting around Donetsk has worsened in recent weeks.
German Chancellor Angela Merkel said last week all 12 points of the September truce have to be fully implemented before the West lifts sanctions on Russia for supporting the rebels.
Moscow has denied all allegations of sending troops and weapons into eastern Ukraine.
In Washington Tuesday, the U.S. announced it is prepared to provide up to $2 billion in loan guarantees to Ukraine if it implements economic reforms spelled out by the International Monetary Fund.
The Treasury Department said the loans will be used to help the Ukrainian government protect the most vulnerable families from the impact of the reforms.