The United States urged Russia on Monday to press Syria to stop bombing opposition forces and civilians in Aleppo and the Damascus suburbs.

The appeal came in a phone call from Secretary of State John Kerry to his Russian counterpart Sergei Lavrov, reported AFP.

“Russia has a special responsibility in this regard to press the regime to end its offensive attacks and strikes that kill civilians, immediately allow relief supplies, as determined solely by the UN, to reach all in need, and to comply completely with the cessation of hostilities,” State  Department spokesman Mark Toner said, according to the news agency.

The United States and Russia are co-partners in the so-called Vienna diplomatic process of the International Support Group for Syria, which met last week in the Austrian capital but made no notable progress.

The 20 world and regional powers taking part in the process have so far failed to turn a fragile cessation of hostilities in Syria, in effect since February 27, into a durable truce between the government and opposition groups.

Toner said the regime of President Bashar Al-Assad was using air strikes and attacks on civilians to gain tactical advantage.

He added the United States is looking to Russia to provide the pressure needed to get the regime “to reconsider the fact that if this keeps up, we may be looking at a complete breakdown of the cessation.”

He said a cessation of hostilities was needed to create an environment for negotiations to begin.

The comments come days after 14 civilians were killed when a barrage of barrel bombs hit the town of Al-Houla and neighboring villages.

Barrel bomb attacks are common in Syria. These are crude weapons — containers packed with explosives and scrap metal that are typically dropped from helicopters.

Their use in Syria’s war has come under fierce criticism by rights groups but Syrian President Bashar Al-Assad has repeatedly denied using barrel bombs and has claimed in interviews that no such weaponry exists.

Indirect negotiations between the government and the opposition have been held three times in Geneva under the auspices of the United Nations, but have made no progress. No date has been set for their resumption.

“Such a (diplomatic) solution will allow all parties to focus on the shared threat posed by Daesh and other terrorists,” Toner said, using an Arabic acronym for the Islamic State (ISIS), the anti-regime militant group that the United States is also fighting.

AFP contributed to this report.