US Secretary of State John Kerry demanded on Wednesday that Russia force Bashar al-Assad’s regime to ground its air force in order to revive hopes of a ceasefire in Syria’s civil war.

Addressing the UN Security Council, including his Russian opposite number Foreign Minister Sergei Lavrov, Kerry said efforts to find peace could yet be salvaged but only if Moscow takes responsibility for recent air strikes.

Kerry said that only Russian and Syrian war planes had been active in areas of northern Syria where on Monday a United Nations aid convoy had been destroyed from the air and on Tuesday a field clinic was bombed.

“I believe that to restore credibility to the process we must move forward to try to immediately ground all air craft flying in those key areas in order to de-escalate the situation and to give a chance for humanitarian assistance to flow unimpeded,” he said.

“And if that happens there’s a chance of giving credibility back to this process,” he said, referring to an agreement he reached with Lavrov in Geneva earlier this month to broker a cessation of hostilities.

“In Geneva, Russia related that Assad was prepared to live by the cessation of hostilities and would accept the idea of not flying over agreed upon areas,” Kerry said.

“But because of what’s happened in the past few days my friends we have no choice but to do that sooner rather than later, move immediately to restore confidence and implement a genuine ceasefire now.”

Moscow has rejected the idea that Russian or Syrian planes carried out Monday’s strike on the UN aid convoy, and Lavrov told the council that there would be “no more unilateral pauses” by Assad’s government forces.

He said that previous breaks in bombing by the government side had only allowed the rebels to re-arm and strengthen their positions and urged UN members to revisit the list of banned terrorist groups excluded from the ceasefire.

“If we can agree on this kind of comprehensive approach, and integrated multi-faced approach, the chances of a cessation of hostilities surviving and being successful will be better,” he argued.