Airstrikes allegedly carried out by Bashar al-Assad warplanes targeted the rebel-held areas in the Syrian city of Aleppo, killing dozens of people, including 14 patients and staff at a Doctors Without Borders hospital.

Citing opposition activists, Fox News reports that Russian jets may have assisted Syrian warplanes in the attacks.

One airstrike destroyed the well-known al-Quds field hospital in the Sukkari district in Aleppo supported by Doctors Without Borders, also known by its French name Médecins Sans Frontières (MSF), killing 14 doctors and patients, including children and one of Syria’s last pediatricians.

The U.K.-based Syrian Observatory for Human Rights, which uses a network of ground sources to monitor the conflict in Syria, reports that a separate strike killed as many as 20 people.

Fox News — citing Doctors Without Borders, opposition activists, and rescue workers — notes that the 24-hour death toll rose to least 61 people.

The airstrikes mark the latest attacks amid an unraveling ceasefire accord and “mounting worries among international relief coordinators and diplomats that Syria’s conflict could be moving onto dangerous new ground with Aleppo as a pivot point,” reports the Washington Post.

Various news outlets point out that it was not immediately clear who carried out the airstrikes. However, Fox News and National Public Radio (NPR) report that opposition activists blame the regime of Syrian dictator Assad.

“Destroyed MSF-supported hospital in Aleppo was well known locally and hit by direct airstrike on Wednesday,” said Doctors Without Borders in a series of tweets, noting that the fatality toll was expected to rise.

United Nations Syria envoy Staffan de Mistura told NPR that the new violence is increasing the risk that warring sides in Syria will return to the “horrible habit” of war.

Although he noted that the ceasefire is holding in some parts of Syria, De Mistura added, ”One Syrian is killed every 25 minutes. And one Syrian is wounded every 13 minutes. So we are very concerned.”

Reuters learned from an unnamed Syrian military source that Assad planes had not been operating in areas where the airstrikes were reported.

“Syria’s army denied reports that the Syrian air force targeted the hospital,” reports Reuters.

“The Russian defense ministry, whose air strikes have swung the war in favor of President Bashar al-Assad, could not immediately be reached for comment,” it adds. “Russia has previously denied hitting civilian targets in Syria where it launched air raids late last year to bolster its ally.”