A postmortem examination of the exhumed body of one of two Palestinian teenagers killed by Israeli forces at a demonstration last month has reportedly identified wounds consistent with live ammunition, despite the Israeli military's denial that it used live rounds that day.

The killings of 17-year-old Nadeem Nawara and 16-year-old Mohammad Salameh caused international outrage and calls from the US for a full investigation after their deaths were caught on video camera footage that made clear the boys posed no threat to Israeli forces at the time of their deaths.

This week Human Rights Watch issued a report suggesting that the killing of the two boys was a war crime. "The wilful killing of civilians by Israeli security forces as part of the occupation is a war crime," said Sarah Leah Whitson, the group's Middle East and North Africa director.

"Israel has a responsibility to prosecute the forces who targeted these teens, and also those responsible for assigning the use of live ammunition to police a demonstration."

Israeli security forces have consistently denied that border police present at the Nakba Day demonstration used live ammunition, insisting only plastic-coated steel rounds were used. It said it was continuing its investigation.

According to leaks from the autopsy team, forensic scientists identified entry and exit wounds as well as bullet fragments in Nadeem's body consistent with live ammunition.

Nadeem's family had sought permission from religious authorities to have the boy's body exhumed and examined on Wednesday by a joint forensic team, including the head of the Palestinian forensic institute and two senior Israeli forensic scientists.

Although Nadeem's father, Siam, told the Guardian on Wednesday that he had not received an official autopsy report, details began leaking out later in the evening.

According to Israeli military sources quoted by Israel's Channel 10, one possibility under examination was that officers interviewed over the killings had lied to military investigators about the fact that live rounds had not been deployed.

The wounds identified by the forensic scientists were said to be consistent with images taken at the hospital and seen by local NGOs showing an exit wound in Nadeem's back from a bullet that had entered his chest close to his nipple.

The case gained international prominence because the video footage of the two teenagers' fatal wounding appeared to clearly demonstrate that neither of the boys posed a threat to Israeli soldiers at the time they were targeted, and that one was walking away from Israeli troops when he was shot.

Anonymous senior Israeli military officials quoted in the local media attempted in the aftermath of the killings to suggest the footage had been forged or a mystery Palestinian gunmen had actually killed the boys – shooting four rounds over a period of more than two hours, apparently without being noticed by several dozen Israeli soldiers and police.