Russia is not bombing hospitals in northern Syria, Kremlin spokesman Dmitry Peskov said Tuesday, calling such reports "unsubstantiated accusations."

"Once again, we categorically reject and do not accept such statements," he said when asked whether Russian planes bombed hospitals in Syria, including one supported by Doctors Without Borders (MSF).

"Especially since every time, those who make such statements are unable to prove in any way their unsubstantiated accusations."

The Kremlin spokesman added that Moscow prefers to rely on "first-hand sources" of information, which he said in this case would be the Syrian government.

Syria's ambassador to Russia, Riad Haddad, on Monday accused the United States of bombing the MSF hospital and said that "Russian warplanes had nothing to do with any of it."

Strikes on hospitals in Idlib and Azaz killed almost 50 civilians including children, according to the United Nations, with UN Secretary General Ban Ki-moon saying the raids violated international law and undermined efforts to end the five-year conflict.

The MSF confirmed its hospital was hit, without assigning blame.

The Syrian Observatory for Human Rights, a British-based monitor, said it suspected that Russian warplanes were behind the attack, based on the location of the raids and the flight patterns and types of planes involved.

The Russian air force has carried out a campaign of strikes to help the ground offensive by the Syrian army since September.

The West accuses it of pursuing the goal of wiping out President Bashar al-Assad's moderate opposition rather than the stated objective to destroy jihadist groups like the IS group.