GENEVA (Reuters) - U.N. Syria envoy Staffan de Mistura called on Russia, Turkey and Iran on Saturday to salvage the tattered ceasefire in Syria that would also help keep peace talks on track.

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Syrian President Bashar al-Assad and his allies, Russia, Iran and Shi’ite militias from nearby countries, are seeking to staunch the biggest rebel assault in months, which began this week in the capital Damascus and the Hama countryside.

“Growing violations in recent days are undermining the ceasefire regime addressed through the Astana meetings, with significant negative consequences for the safety of Syrian civilians, humanitarian access and the momentum of the political process,” de Mistura said in a statement on the third day of peace talks.

The U.N. envoy said he had written to the foreign ministers of Russia, Iran, and Turkey -- the three guarantors of the ceasefire that came into effect on December 30, urging them to “undertake urgent efforts to uphold the ceasefire regime.”

Nasr Hariri, lead negotiator of the opposition High Negotiations Committee (HNC), said after meeting the U.N. envoy on Saturday that 15 civilians were killed and 70 wounded in the bombing of a market in Hamouriya in the Damascus countryside.

“I am talking about the killing machine and terror machine of the Assad regime,” Hariri said, reading the names of victims of the “massacre” to reporters in Geneva.

He also accused the government of the forced displacement of civilians and imposing “demographic change”.

“Any political solution would not be meaningful or credible if it was not accompanied with a real ceasefire on the ground,” Hariri said. “If we don’t have that real ceasefire then things will deteriorate very badly.”

Hariri said the opposition delegation and de Mistura had discussed political transition, one of four themes that the two sides have agreed to as the agenda for this fifth round.

The government delegation led by Syrian ambassador Bashar al-Ja’afari presented the U.N. mediator on Saturday with a paper on fighting terrorism, another agenda item that Damascus views as the priority.

The Syrian Network for Human Rights said in a report it had documented 948 civilians killed, including 192 children and 91 women, “largely at the hands of the Syrian regime and Russia” since the last round of Geneva peace talks ended on March 3.