Terrified children were forced to shelter in the basement yesterday as the shelling yielded the highest single civilian toll since a tenuous ceasefire was struck between Kiev and pro-Russian separatists last month.
Clashes have raged for days at several flashpoints around the region, with both sides blaming the other for violating the agreement that commits them to withdrawing weapons and establishing a buffer zone.
The United States has decried the violence while the European Union decided Tuesday to keep Russian sanctions in place, maintaining pressure on Moscow in the worst East-West standoff since the Cold War.
Four adults were killed, it said in a statement. Six more died when another shell struck a minibus at a bus stop about 500 metres from the school.
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"I was going to open the doors to let passengers in and out when it happened," the driver of the vehicle, Mikhail Drobotun, told AFP as he sat on a gurney in a local clinic where dozens of the injured were being treated as artillery boomed in the background.
AFP correspondents saw pools of blood at the deserted two-storey school, which is located about four kilometres away from the government-held Donetsk airport in an area that is frequently hit by shelling.
A source in Donetsk city hall told AFP that the strike happened right after the school's 70 pupils lined up for an assembly to mark the first day of class - held nationally on September 1 but pushed back by rebel authorities because of the conflict.
Official rebel website DNR Today blamed the attack on Ukrainian artillery, claiming rebels do not possess the relevant weaponry and saying two security guards at the school were among the victims.
Amnesty International in a statement urged Ukrainian and rebel forces to "immediately end indiscriminate attacks in residential areas", adding that both sides shared the blame for endangering civilians.