At least 30 Ukrainian soldiers are believed to have been killed by a rocket strike in the east of the country.
According to Ukrainian officials a barrage of Grad rockets has been fired by pro-Russia separatists on government troops in the city of Lugansk.
"So far, the information available speaks of up to 30 [dead]. The number of casualties may increase," said Zoryan Shkyryak, an adviser to Ukrainian Interior Minister Arsen Avakov.
"I think a response will not be slow in coming after this bloody terrorist act," he added.
The rocket strike comes one day after the Ukrainian government launched an "all-out" attack on separatists near the city of Donetsk and the neighbouring city of Lugansk.
The attack follows a warning to the Pro-Russian separatists by the Ukrainian government that it planned to regain control of the areas.
The Ukrainian Donetsk and Lugansk regions, on the border with Russia, held disputed referendums on 11 May in which most voters supported independence from Ukraine.
Violence escalated in the region following the outbreak of unrest between pro-government forces and rebels in April.
Both sides have been accused of violating a 10-day ceasefire offered by newly-elected Ukrainian president Petro Poroshenko on 20 June.
At least 49 soldiers were killed in Lugansk by another attack carried out in the separatists in mid-June.