Donetsk, Ukraine: Ten Ukrainian civilians were killed and nearly 20 injured on Tuesday when a long-range Grad rocket apparently fired by pro-Russian insurgents hit an intercity bus in the separatist east.

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 was the deadliest attack on civilians since the rival sides signed a much-maligned September 5 ceasefire that only partially stemmed the fighting and did little to resolve the insurgents’ independence claims.

Tuesday’s strike also damaged Ukrainian President Petro Poroshenko’s efforts to set up a peace summit where his Russian counterpart Vladimir Putin could personally sign a truce to try to end the ex-Soviet republic’s nine-month war.

German Chancellor Angela Merkel — the West’s main mediator in Europe’s deadliest conflict since the Balkan wars of the 1990s — argues that such a meeting would be premature with violence still raging daily across Ukraine’s Russian-speaking rustbelt.

Diplomatic talks in Berlin on Monday confirmed that no summit would be held on the crisis in the short term.

Both Ukrainian military and regional police said that the death toll from Tuesday’s incident comprised six women and four men.