MOSCOW — Syria has told Russia that a mortar attack that killed five civilians across the border in Turkey was a "tragic accident" that will not be repeated, Russian Foreign Minister Sergei Lavrov was quoted as saying on Thursday.

RIA Novosti news agency said Lavrov had also told reporters during a visit to Islamabad that Russia had urged Syria to acknowledge this in public.

"Through our ambassador to Syria, we have spoken to the Syrian authorities who assured us . . . that what happened at the border with Turkey was a tragic accident, and that it will not happen again," RIA quoted Lavrov as saying. "We think it is of fundamental importance for Damascus to state that officially."

Russia, which is an ally of Syria and opposes international efforts to oust President Bashar al-Assad, also called for restraint after a Syrian mortar bomb hit a residential neighborhood of the Turkish border town of Akcakale.

Turkey hit back with artillery fire, in which activists and security services said several Syrian soldiers had been killed. They also said Turkey was reinforcing its troops along the border.

Lavrov said Syria and Turkey should establish a channel to discuss border issues, including the flow of refugees from Syria. He said the conflict in Syria had taken on a "cross-border dimension".

The violence is the most serious cross-border escalation of the 18-month-old uprising in Syria.

U.N. Security Council members had hoped to issue a non-binding statement on Wednesday that would condemn the mortar attack "in the strongest terms" and demand an end to violations of Turkey's territorial sovereignty. But Russia asked for a delay, diplomats said.

About 30,000 people have been killed across Syria, activists say, in a conflict with growing sectarian overtones that threatens to draw in regional Sunni and Shi'ite Muslim powers.