It comes a day after calls for aircrafts to be grounded so a ceasefire can be agreed upon
Rebel-held areas of Aleppo in Syria have suffered their most intense airstrikes in months, according to reports.
The development comes a day after John Kerry called for Russian and Syrian government aircraft to be grounded in parts of the country so a ceasefire agreement can be extended.
Speaking at the UN, the US Secretary of State said it was the last chance to salvage the collapsing deal and find a way "out of the carnage" but the future of the country was "hanging by a thread".
He also said an airstrike on a Red Cross convoy in Aleppo on Monday, which killed 20 aid workers delivering humanitarian supplies, raised "profound doubt" as to whether Russia and the Syrian government were committed to upholding the cessation of hostilities.
The Syrian military has not yet commented on the claims about the latest intense airstrikes which were made by rebel officials and a monitoring group.
Meanwhile, the International Syria Support Group, made up of about 20 countries with interests in the conflict, will meet later today and resume diplomatic efforts to save the troubled ceasefire deal.
Despite tension between the US and Russia, all key players in the civil war will gather in New York.
After halting aid operations in the wake of the attack on the convoy, the UN has said it is ready to resume humanitarian deliveries.
Sounding a cautious note, Mr Kerry told reporters late on Wednesday that "it's going to be difficult. We'll see what people are willing to do."