The Russian Air Force has repeatedly bombed hospitals in Syria in order to crush the last pockets of resistance to President Bashar al-Assad, according to an investigation by The New York Times.

An analysis of previously unpublished Russian Air Force radio recordings, plane-spotter logs and witness accounts allowed The New York Times to trace bombings of four hospitals in just 12 hours in May and tie Russian pilots to each one.

The 12-hour period beginning on 5 May represents a small slice of the air war in Syria, but it is a microcosm of Russia’s four-year military intervention in Syria’s civil war. A new front in the conflict opened this week, when Turkish forces crossed the border as part of a campaign against a Kurdish-led militia.

Russia has long been accused of carrying out systematic attacks against hospitals and clinics in rebel-held areas as part of a strategy to help Assad secure victory in the eight-year-old war.

Physicians for Human Rights, an advocacy group that tracks attacks on medical workers in Syria, has documented at least 583 such attacks since 2011, 266 of them since Russia intervened in September 2015. At least 916 medical workers have been killed since 2011.

Syrian government offensive in Idlib Show all 8 1 /8 Syrian government offensive in Idlib Syrian government offensive in Idlib Syrians, including a member of the White Helmets civil defence rescue teams, run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians look for survivors in a heavily damaged area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib This photo provided by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows Civil Defense workers searching for victims under the rubble of a destroyed building after an airstrike by Syrian government forces, Maaret al-Numan, a town in south Idlib, Syria, Thursday, May 30, 2019. Rescue workers and activists say at least five, including three from the same family, have been killed in Syria's last rebel stronghold when warplanes targeted the building they live in, leveling it. AP Syrian government offensive in Idlib A Syrian girl cries as she runs for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrians run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib A man rescues a child while members of the Syrian civil defence known as the White Helmets search the area for survivors following following a reported air strike by regime forces and their allies on Maaret al-Noman in Syria's southwestern Idlib province. AFP/Getty

The New York Times assembled a large body of evidence to analyse the hospital bombings on 5-6 May.

Social media posts from Syria, interviews with witnesses, and records from charities that supported the four hospitals provided the approximate time of each strike. The New York Times obtained logs kept by flight spotters on the ground who warn civilians about incoming airstrikes and cross-checked the time of each strike to confirm that Russian warplanes were overhead. The New York Times then listened to and deciphered thousands of Russian Air Force radio transmissions, which recorded months’ worth of pilot activities in the skies above northwestern Syria. The recordings were provided by a network of observers who insisted on anonymity for their safety.

Spotter logs from 5-6 May put Russian pilots above each hospital at the time they were struck, and Air Force audio recordings from that day feature Russian pilots confirming each bombing. Videos obtained from witnesses and verified by The New York Times confirmed three of the strikes.

Recklessly or intentionally bombing hospitals is a war crime, but proving culpability amid a complex civil war is extremely difficult, and until now, Syrian medical workers and human rights groups lacked proof.

Russia’s position as a permanent member of the United Nations Security Council has shielded it from scrutiny and made UN agencies reluctant to accuse the Russian Air Force of responsibility.

“The attacks on health in Syria, as well as the indiscriminate bombing of civilian facilities, are definitely war crimes, and they should be prosecuted at the level of the International Criminal Court in The Hague,” said Susannah Sirkin, director of policy at Physicians for Human Rights. But Russia and China “shamefully” vetoed a Security Council resolution that would have referred those and other crimes in Syria to the court, she said.

The Russian government did not directly respond to questions about the four hospital bombings. Instead, a Foreign Ministry spokesman pointed to past statements saying that the Russian Air Force carries out precision strikes only on “accurately researched targets.”

The UN secretary-general, Antonio Guterres, opened an investigation into the hospital bombings in August. The investigation, still ongoing, is meant in part to determine why hospitals that voluntarily added their locations to a UN-sponsored deconfliction list, which was provided to Russia and other combatants to prevent them from being attacked, nevertheless came under attack.

Syrian health care workers said they believed the UN list actually became a target menu for Russian and Syrian air forces.

Stephane Dujarric, a spokesman for the secretary-general, said in September that the investigation — an internal board of inquiry — would not produce a public report or identify “legal responsibility.” Vassily Nebenzia, the Russian permanent representative to the UN, cast doubt on the process shortly after it was announced, saying he hoped the inquiry would not investigate perpetrators but rather what he said was the UN’s use of false information in its deconfliction process.

From 29 April to mid-September, as Russian and Syrian government forces assaulted the last rebel pocket in the northwest, 54 hospitals and clinics in opposition territory were attacked, the UN human rights office said. At least seven had tried to protect themselves by adding their location to the deconfliction list, according to the World Health Organisation.

On 5-6 May, Russia attacked four. All were on the list.

The first was Nabad al Hayat Surgical Hospital, a major underground trauma centre in southern Idlib province serving about 200,000 people. The hospital performed on average around 500 operations and saw more than 5,000 patients a month, according to Syria Relief and Development, the US-based charity that supported it.

Nabad al Hayat had been attacked three times since it opened in 2013 and had recently relocated to an underground complex on agricultural land, hoping to be protected from airstrikes.

At 2:32pm on 5 May, a Russian ground control officer can be heard in an Air Force transmission providing a pilot with a longitude and latitude that correspond to Nabad al Hayat’s exact location.

At 2:38pm, the pilot reports that he can see the target and has the “correction,” code for locking the target on a screen in his cockpit. Ground control responds with the green light for the strike, saying, “Three sevens.”

At the same moment, a flight spotter on the ground logs a Russian jet circling in the area.

At 2:40pm, the same time the charity said that Nabad al Hayat was struck, the pilot confirms the release of his weapons, saying, “Worked it.” Seconds later, local journalists filming the hospital in anticipation of an attack record three precision bombs penetrating the roof of the hospital and blowing it out from the inside in geysers of dirt and concrete.

The staff of Nabad al Hayat had evacuated three days earlier after receiving warnings and anticipating a bombing, but Kafr Nabl Surgical Hospital, 3 miles northwest, was not as lucky.

A doctor who worked there said that the hospital was struck four times, beginning at 5:30pm. The strikes landed about five minutes apart, without warning, he said, killing a man who was standing outside and forcing patients and members of the medical staff to use oxygen tanks to breathe through the choking dust.

A spotter logged a Russian jet circling above at the time of the strike, and in another Russian Air Force transmission, a pilot reports that he has “worked” his target at 5:30pm, the time of the strike. He then reports three more strikes, each about five minutes apart, matching the doctor’s chronology.

Russian pilots bombed two other hospitals in the same 12-hour span: Kafr Zita Cave Hospital and Al Amal Orthopedic Hospital. In both cases, spotters recorded Russian Air Force jets in the skies at the time of the strike, and Russian pilots can be heard in radio transmissions “working” their targets at the times the strikes were reported.

Since 5 May, at least two dozen hospitals and clinics in the rebel-held northwest have been hit by airstrikes. Syrian medical workers said they expected hospital bombings to continue, given the inability of the UN and other countries to find a way to hold Russia to account.

“The argument by the Russians or the regime is always that hospitals are run by terrorists,” said Nabad al Hayat’s head nurse, who asked to remain anonymous because he feared being targeted. “Is it really possible that all the people are terrorists?

“The truth is that after hospitals are hit, and in areas like this where there is just one hospital, our houses have become hospitals.”