Twelve Ukrainian civilians were killed and nearly 20 injured on Tuesday as a long-range Grad rocket hit a bus in Ukraine’s war-torn east.
Donetsk regional officials blamed pro-Russian rebels but a report said rebel leaders had denied involvement.
Local police said the rocket appeared to have gone astray after being aimed by the gunmen at a checkpoint set up by government soldiers on the main highway connecting the rebel stronghold of Donetsk with Ukraine's southeastern coast on the Sea of Azov.
The incident, which was condemned by the UN Security Council, was the deadliest attack on civilians since a ceasefire agreement signed Sept 5.
The latest incident comes after peace talks mooted for this week were scrapped.
President Poroshenko said in December that he planned to meet Russian President Vladimir Putin in Astana, capital of Kazakhstan, on 15 January alongside the German and French leaders.
But following talks in Berlin on Monday, foreign ministers for the four countries said that "further work needs to be done" before a summit could be held.