An adviser to Ukrainian Interior Minister Arsen Avakov says separatist rebels in the east may have killed as many as 30 government soldiers in a rocket attack.
Zoryan Shkyryak is quoted by Reuters as saying the rebels used Grad multiple-rocket launchers in their assault.
"Up to 30 [were killed]," Shkyryak said. "It is not excluded that the number of victims will rise because these bloodthirsty scum despicably shot from Grad [rocket] systems and there is destruction."
Shkyryak said authorities would react swiftly to punish the perpetrators.
"They will be destroyed or captured and be made answerable to Ukrainian law," he said.
The news comes amid a renewed government offensive against the rebels.
Elsewhere, at least four Ukrainian troops and five coal miners were reported killed.
The miners were killed on July 11 after the bus they were traveling in reportedly came under mortar fire.
Two soldiers and at least one border guard were killed after their armored vehicle drove over a land mine near the Russian border.
Another soldier was killed in an incident near the town of Karlivka in the Donetsk region.
Military spokesman Vladyslav Seleznyov said at least 50 separatist fighters had been killed in the last few days following government air strikes.
There has been no immediate comment from the rebels.
Seleznyov also said government forces had regained the town of Siversk, east of Slovyansk, when separatists fled on June 10. A separatist statement confirmed the government's version, saying it was "more or less correct."
Seleznyov said Ukrainian troops guarding Donetsk's international airport came under mortar fire on July 10 but the rebel attack was repelled.
Rebels seized the airport in May but Ukrainian forces recaptured it after a battle in which dozens of people died.
The military said three more Ukraiian soldiers died on July 9 in different parts of the east.
The Ukrainian military says it has a plan to deliver a "nasty surprise" to the heavily armed separatists holed up in Donetsk, a city of 900,000, after being pushed out of their bastion in Slovyansk last weekend.
WATCH: Armed pro-Russian separatists continued to strengthen their positions in and around Donetsk on July 10 despite a warning from Kyiv that a plan was now in place to take back the territory they occupied.
Meanwhile, French President Francois Hollande and German Chancellor Angela Merkel urged Russian leader Vladimir Putin by telephone to "exert all necessary pressure on the separatists to bring them to negotiate effectively."
A French presidency statement said they also asked him to take concrete steps to ensure control of the border.
Separately, the White House said on July 10 that Ukrainian President Petro Poroshenko told U.S. Vice President Joe Biden that Moscow and the rebels had refused multiple proposals by Kyiv for venues to negotiate a cease-fire.
Moscow protested to Kyiv on July 10 after it said the Ukrainian military fired on a Russian border checkpoint. The Foreign Ministry said it was not the first time the border post at Gukovo had come under fire.
Moscow is under sanctions by the United States and the European Union over the Ukraine crisis but denies it is supporting the rebels in the Russian-speaking east of Ukraine.
The EU has reportedly decided to add 11 new names to the list, likely to take effect on July 12.
Russia condemned the plan to extend the list.
Poroshenko, in a phone conversation with Merkel on July 10, said Russia was violating international norms by holding a Ukrainian woman pilot, Nadiya Savchenko, and called for her release.
Savchenko, 33, was seized by rebels in June while she was fighting with a pro-government militia near Luhansk.
But she has now turned up in detention in Russia and Moscow says she has been charged with involvement in the deaths of two Russian reporters killed near Luhansk.
With reporting by AP, AFP, Reuters, and BBC