Israel’s military has backtracked on its account of the killing of 15 Palestinian medics in Gaza last month after footage contradicted its claims that their vehicles did not have emergency signals on when Israeli troops opened fire.

The military said initially it opened fire because the vehicles were “advancing suspiciously” on nearby troops without headlights or emergency signals. An Israeli military official, speaking on condition of anonymity in line with regulations late on Saturday, said that account was “mistaken”.

The almost seven-minute video, which the Palestine Red Crescent Society (PRCS) said on Saturday was recovered from the phone of Rifat Radwan, one of the men killed, appears to have been filmed from inside a moving vehicle. It shows a red fire engine and clearly marked ambulances driving at night, using headlights and flashing emergency lights.

The vehicle stops beside another that has driven off the road. Two men get out to examine the stopped vehicle, then gunfire erupts before the screen goes black.

Fifteen Palestinian paramedics and rescue workers, including at least one UN employee, were killed in the incident in Rafah on 23 March, in which the UN said Israeli forces shot the men “one by one” and then buried them in a mass grave.

The Israel Defense Forces said the incident was still under investigation. It added: “All claims, including the documentation circulated about the incident, will be thoroughly and deeply examined to understand the sequence of events and the handling of the situation.”

The official said the initial report received from the field did not describe lights but that investigators were looking at “operational information” and were trying to understand whether this was due to an error by the person making the initial report.

“What we understand currently is the person who gives the initial account is mistaken. We’re trying to understand why,” the official added.

According to the UN Office for the Coordination of Humanitarian Affairs (Ocha), the PRCS and civil defence workers were on a mission to rescue colleagues who had been shot at earlier in the day, when their clearly marked vehicles came under heavy Israeli fire in the Tel al-Sultan area of Rafah. A Red Crescent official in Gaza said there was evidence of at least one person being detained and killed, as the body of one of the dead had been found with his hands tied.

The shootings happened one day into the renewed Israeli offensive in the area close to the Egyptian border after the breakdown of a two-month ceasefire between Israel and Hamas. Another Red Crescent worker on the mission, Assad al-Nassasra, is still reported missing and the organisation has asked the Israeli military for information on his whereabouts.

A survivor of the incident, the Palestinian Red Crescent paramedic Munther Abed, has said he saw Nassasra being led away blindfolded by Israeli troops.

The 27-year-old volunteer was in the back of the first ambulance to arrive on the scene of an airstrike in the Hashashin area of Rafah before dawn on 23 March when the vehicle came under intense Israeli fire.

His two Red Crescent colleagues sitting in the front were killed but he survived by throwing himself to the floor of the vehicle. “The door opened, and there they were – Israeli special forces in military uniforms, armed with rifles, green lasers and night-vision goggles,” Abed told the Guardian. “They dragged me out of the ambulance, keeping me face down to avoid seeing what had happened to my colleagues.”

Adeb was detained for several hours before being released.

The UN and Palestinian Red Crescent have demanded an independent inquiry into the killing of the paramedics.

Israeli media briefed by the military have reported that troops had identified at least six of the 15 dead as members of militant groups and killed a Hamas figure named Mohammed Amin Shobaki.

None of the 15 killed has that name and no other bodies are known to have been found at the site. The official declined to provide any evidence or detail of how the identifications were made, saying he did not want to share classified information.

“According to our information, there were terrorists there but this investigation is not over,” he told reporters.

Abed – a volunteer for 10 years – was adamant there were no militants travelling with the ambulances.

Jonathan Whittall, the interim head of Ocha in the occupied Palestinian Tterritory, dismissed allegations that the people who died were Hamas militants, saying staff had worked with the same medics previously in evacuating patients from hospitals and other tasks.

“These are paramedic crews that I personally have met before,” he said. “They were buried in their uniforms with their gloves on. They were ready to save lives.”

The Israeli military official said the troops had informed the UN of the incident on the same day and initially covered the bodies with camouflage netting until they could be recovered, later burying them when the UN did not immediately collect the bodies.

The UN confirmed last week it had been informed of the location of the bodies but that access to the area was denied by Israel for several days. It said the bodies had been buried alongside their crushed vehicles – clearly marked ambulances, a fire truck and a UN car.