An airstrike on a Doctors Without Borders hospital Monday in northern Syria was deliberate and carried out by either Syrian regime or Russian warplanes, the medical nongovernmental organization said. The missiles struck four hospitals and a nearby school, leaving at least 23 people dead, Reuters reported.

While initial estimates had put the death toll around a dozen people, as many as 50 people may have been killed in the attack, according to the United Nations, the New York Times reported. The U.N. condemned the airstrikes as "blatant violations of international law," Agence France-Presse reported.

The hospital in Maarat al-Numan was hit by four missiles minutes apart, and the quick succession "leads us to believe that ... it wasn't an accidental attack, that it was deliberate," said Sam Taylor, a spokesman for Doctors Without Borders operations in Syria, the BBC reported.

Representatives from the NGO said the attack was carried out by either Russian or Syrian aircraft. U.S. and allied military were not operating near the region of the airstrikes at the time, according to a tweet from the verified account of Col. Steve Warren, spokesman for the U.S. operation against the terror organization known as the Islamic State group.

The hospital was reduced to rubble, leaving at least seven dead and many more wounded. Another nearby hospital hit in the airstrikes suffered at least five fatalities, the New York Times reported. The local population of 40,000 people no longer has access to healthcare, a representative from the medical charity told the Times.

It’s not the first time a Doctors Without Borders hospital has been either accidentally or purposefully hit with airstrikes in recent months. The widely publicized Oct. 3, 2015, shelling of a hospital run by the charity in Kunduz, Afghanistan, caused public outrage after the U.S. admitted it was responsible but called it an accident. The airstrikes on Kunduz left 42 people dead, and many more wounded.

As violent conflict in Syria has continued to rage, civilians’ access to healthcare has grown increasingly sparse. At least 654 medical personnel have been killed since the start of the armed conflict in 2011, according to a September 2015 report from the World Health Organization.