The Russian air force has repeatedly bombed hospitals in Syria to try and 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An analysis of previously unpublished Russian air force radio recordings, plane spotter logs and witness accounts has allowed The Times to trace the bombings of four hospitals in just 12 hours in May and tie Russian pilots to each one.

The 12-hour period beginning May 5 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, of which 266 happened since Russia intervened in September 2015. At least 916 medical workers have been killed since 2011.

The Times has assembled a large body of evidence to analyse the hospital bombings on May 5 and 6.

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 Times obtained logs kept by flight spotters on the ground who warn civilians about incoming air strikes, and crosschecked the time of each strike to confirm that Russian warplanes were overhead.

We then listened to and deciphered thousands of Russian air force radio transmissions, which recorded months’ worth of pilot activities in the skies above northwest Syria. The recordings were provided to The Times by a network of observers who insisted on anonymity for their safety.

The spotter logs from May 5 and 6 put Russian pilots above each hospital at the time they were struck, and the air force audio recordings from that day feature Russian pilots confirming each bombing. Videos obtained from witnesses and verified by The 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 UN Security Council has shielded it from scrutiny and made UN agencies reluctant to accuse its 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.

Moscow did not directly respond to questions about the four hospital bombings. Instead, a foreign ministry spokesperson pointed to past statements saying that the Russian air force carries out precision strikes only on “accurately researched targets.”

The UN secretary-general, António Guterres, opened an investigation into the hospital bombings in August. The probe, still going on, is meant partly to determine why hospitals that voluntarily added their locations to a UN-sponsored de-confliction list, which was provided to Russia and other combatants to prevent attacks on them, nevertheless came under attack.

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

Stéphane Dujarric, a spokesperson 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 the perpetrators but rather what he described as the UN’s use of false information in its de-confliction process.

From April 29 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 de-confliction list, according to the World Health Organisation.

On May 5 and 6, 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 United States-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 air strikes.

At 2.32pm on May 5, 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, three miles northwest, was not as lucky.

A doctor who worked there said 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 Orthopaedic 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 May 5, at least two dozen hospitals and clinics in the rebel-held northwest have been hit by air strikes. Syrian medical workers said they expected the 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?”

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