One strike hit near a hospital in Idlib and forced the facility to close. Workers turned the underground ambulance area into a trauma center, the source said. This is the only part of the hospital in use.
An adjacent hospital -- the newly built Ibn Sina Center -- was also damaged in the Monday strike but was unoccupied at the time, he said.
The death toll is expected to rise as rescuers continue to pull victims from the rubble.
Video posted by the Syrian Civil Defense, a volunteer search-and-rescue operation, showed responders digging through the rubble, searching for survivors. One rescuer could be seen lifting the body of a young boy in his arms, his face covered in dust.
Dangers for doctors in Syria
Syria has been one of the more dangerous places in the world to work in for doctors.
In these latest airstrikes in Idlib, a 23-year-old aid worker with Save the Children died while trying to rescue families trapped under the rubble. He was killed in a subsequent strike along with several others, the organization said.
Medecins Sans Frontieres, also known as Doctors Without Borders, reported evacuating its staff
and patients from a hospital near Azaz, where ISIS launched an offensive. The hospital remains closed.
In April, at least 50 people were killed when a pediatric hospital in Aleppo was hit, drawing international condemnation.
Two doctors, two nurses, one guard and one maintenance worker were killed, according to Doctors Without Borders
"Syria is the most dangerous place in the world to be a doctor," says the website of Medics Under Fire
, a nonprofit group that campaigns to protect hospitals in war zones. "Medical workers are systematically targeted by the Syrian government in a bid to weaken the civilian areas not under its control."
As of December, 57% of public hospitals and 51% of public health centers in Syria were either partially functioning or closed down, according to the World Health Organization.
About 15,000 doctors have fled Syria, according to Medics Under Fire, and those that remain risk their lives on a daily basis.
Hospitals had long been considered safe zones during the fighting, and medical workers will often give hospital locations to all parties to avoid being hit.
But hospitals in Syria have increasingly been caught in the crossfire, with multiple sides accused of hitting medical facilities. The regime of Bashar al-Assad has been consistently accused of targeting hospitals, which is a war crime.
Militias launch offensive against ISIS in Manbij
Elsewhere in northern Syria, thousands of U.S.-backed Kurdish and Arab fighters known as the Syrian Democratic Forces have launched an offensive on the "critical and strategic" area of Manbji, a senior administration official said.
"Manbij is a hub of [ISIS] external operations. It is a critical supply node on route to Raqqa," the official said.
"Cut them off there and they're totally isolated in Raqqa. So it's critical, strategic, and we have now launched an operation long in planning to go after it."
The operation aims to keep ISIS fighters from crossing into Turkey and ensure that foreign fighters cannot enter Syria.
The SDF has made some progress, retaking 16 small villages and farms from ISIS, according to the Syrian Observatory for Human Rights.
A U.S. official said a small number of U.S. specials ops are advising the SDF, adding that the coalition is providing air support while Turkey is helping from across the border.