By Richard Balmforth
KIEV (Reuters) - A passenger bus came under heavy fire in eastern Ukraine on Tuesday, killing at least 11 people, Ukrainian authorities said, and fighting intensified around the international airport in the city of Donetsk as separatists tried to oust government forces.
The latest violence flared after Russia, Ukraine, France and Germany scrapped plans for a summit in Kazakhstan this week because of the failure to implement a four-month-old ceasefire agreement.
Ukrainian President Petro Poroshenko condemned the bus attack as an act that "chilled the heart" and said the forces of the separatist Donetsk and Luhansk People's Republics were to blame.
"These deaths are on the conscience of the DNR and LNR gangs and on those who stand behind them," he said in a statement in which he promised to sign a decree on Wednesday to more troops for the front.
A senior official from the Organization for Security and Cooperation in Europe (OSCE) called for maximum restraint from all sides in the Ukraine conflict, which has killed more than 4,700 people since last April.
"Over the past 24 hours the situation has significantly deteriorated, especially near the Donetsk airport," said Ertugrul Apakan, head of the OSCE special monitoring mission in Ukraine.
The war between Ukrainian forces and pro-Russian rebels broke out soon after Russia annexed Crimea last year, creating the worst crisis in East-West relations since the Cold War. Western governments accuse Russia of backing the separatists, including by sending in troops, which it denies.
Photographs showed the bus peppered by holes, as were seats inside it. A long trail of blood marked the road beside it near the town of Volnovakha.
A regional Ukrainian administration spokesman said the bus was attacked by rebels using Grad rocket launchers while it was carrying civilians through a government checkpoint.
Separatists denied responsibility and said the bus had been attacked by small arms fire rather than a missile or shell.
Reports from Donetsk said a significant part of the airport's control tower - already a wrecked hulk with cabling and concrete dangling from it after months of shelling - had been destroyed.
The Sergei Prokofiev airport complex, opened to great fanfare by the now ousted president Viktor Yanukovich to mark the Euro 2012 soccer championship, has disintegrated under months of fire.
After a night of attacks from separatists using Grad missile launchers, the rebels began firing from tanks on the airport's new terminal, which was still being held by Ukrainian government forces, the Kiev military said in a statement.
"The Russian military and the terrorists have deliberately chosen the tactic of escalation of tension," military spokesman Andriy Lysenko told journalists. One Ukrainian soldier was killed and 10 wounded in overnight fighting.
Although it has not functioned since the onset of hostilities last April, with its runways cratered by shell holes, the airport has symbolic value for both sides. Government forces have repelled repeated rebel attempts to dislodge them.
In the capital Kiev, a parliamentary deputy said government forces had been given an ultimatum by the rebels to pull out of the airport by 5 p.m. (10.00 a.m. EST) "or face destruction".
This could not be confirmed from the separatist side.
The Russian, German and French government leaders had been invited to talks on Thursday in the Kazakh capital Astana by Ukrainian President Petro Poroshenko.
But the four countries' foreign ministers said after meeting in Berlin on Monday that the failure to implement the ceasefire deal and the need to agree on how to deliver aid and release prisoners meant "further work needs to be done" before a summit is held. There was no indication when it might now take place.
(Reporting by Stephen Brown in Berlin, Elizabeth Piper and Gabriela Baczynska in Moscow, Pavel Polityuk, Natalia Zinets and Alessandra Prentice in Kiev; Writing by Richard Balmforth; Editing by Mark Trevelyan)