ukraine bus
People look at a city bus was hit by an artillery shell in DonetskAlexander Ermochenko/Reuters

A mortar shell hit a trolleybus stop in the eastern Ukrainian city of Donetsk, killing up to 13 people, Russian media have reported citing separatist leaders in the restive region.

Dozens were reportedly wounded in the explosion that came a day after diplomats from Russia and Ukraine agreed to pull back heavy weapons from the conflict frontline.

It was not immediately clear who was responsible for the attack.

The Guardian's Shaun Walker tweeted:

Ukrainian forces and Russian-backed separatists have been trading blame for a series of similar incidents that killed dozens of civilians, as fighting flared up over the last few days in the separatist regions of Donetsk and Luhansk.

Last week, 13 civilians were killed as a commuter bus from Donetsk was hit by a shell in the near town of Volnovakha.

Ukrainian president Petro Poroshenko claimed the deaths were a direct consequence of Russian heavy artillery fire in support of separatist rebels.

"For me this is a symbol, a symbol of the terroristic attack against my country," he said addressing the World Economic Forum in Davos, Switzerland. Poroshenko also said some 9,000 Russian troops are currently inside Ukrainian territory.

Rebels have denied responsibility for the bus attack, while Moscow has long denied any involvement in the conflict, despite evidence suggesting the contrary.

More than 4,700 people have died because of the conflict since April last year.