Eleven people in Kievm Ukraine died as a bus was attacked Ukrainian, authorities said. Separatists have denied responsibility for the attacks. At the same time, the battle between separatist and government forces became even more deadly around Donetsk, which is home to an international airport. 

"The Russian military and the terrorists have deliberately chosen the tactic of escalation of tension," Ukrainian military representative Andriy Lysenko said according to Reuters.

A government official said that serpatists gave their forces an ultimatum to vacate the airport for 5 p.m "or face destruction." After that deadline passed, the fighting ratcheted up even though the airport itself has not been functional for months; it's viewed as a prized piece by both sides. 

Ukrainian President Petro Poroshenko said that the attack "chilled the heart."

"These deaths are on the conscience of the DNR and LNR gangs and on those who stand behind them," he said.

This new round of violence comes as Russia, Ukraine, France, and Germany dropped plans to attend a summit in Kazakhstan this week because there has been no success in the enforcement of a ceasefire pact that was reached four months ago.

The leaders of the governments that were supposed to attend the peace summit were invited by Ukrainian President Poroshenko.

The foreign ministers for each nation, however, said on Monday that since there was no success with the ceasefire agreement or an agreement on how to release prisoners or deliver aid, "further work needs to be done." Once these goals are met, a new summit could be scheduled and attended by representatives of these same nations.

The conflict between Ukrainian forces and pro-separatist rebels began after Russia annexed Crimea last year. Russia has been accused of supporting the separatists, though the nation has denied any involvement in the violence.