Mohamed Abu Khdeir, the Palestinian teenager who was kidnapped and murdered on Wednesday in a suspected revenge killing by Israeli extremists, was burned alive after suffering a head injury, the Palestinian attorney general has claimed

The allegation is said to be based on initial postmortem findings that discovered soot deposits in his lungs suggesting he was still breathing when he was set on fire. The shocking details, if confirmed, would seem likely to exacerbate already toxic tensions.

The reports emerged as Egypt tried to conclude a ceasefire deal between Hamas in Gaza and Israel. But it appeared not to have taken hold, with fresh reports of rocket fire into Israel from the coastal strip.

The murder of 17-year-old Khdeir, who was buried on Friday in a highly charged funeral after his abduction outside a mosque next to his home in the early hours of Wednesday morning, has prompted days of serious rioting in Palestinian neighbourhoods of East Jerusalem, which then spread to Israeli-Arab towns.

"The direct cause of death was burns as a result of fire and its complications," attorney general Mohammed al-A'wewy told the Palestinian official news agency, Wafa, late on Friday. Israeli officials have yet to release their findings from the postmortem on the body.

Tensions have risen after three Israeli teenagers were kidnapped on 12 June and later found dead in the occupied West Bank. That has been followed by an outbreak of racist incitement on Israeli social media sites, street attacks and Khdeir's murder, a suspected revenge attack.

Saber al-Aloul, director of the Palestinian forensic institute, attended the postmortem carried out by Israeli doctors in Tel Aviv. A'wewy said Aloul had reported that fire-dust material had been found in Khdeir's respiratory canal, which meant "the boy had inhaled this material while he was burned alive". Burns covered 90% of his body.

The discovery of the youth's body in a forest on the outskirts of Jerusalem has prompted the worst riots in the holy city in recent memory. The violence spread to northern Arab towns on Saturday morning, an Israeli police spokeswoman, Luba Samri, said. Protesters there threw stones at passing cars, burned tyres and hurled fire bombs at police, who responded with teargas and stun grenades. More than 20 people were arrested.

At Khdeir's funeral, furious Palestinians chanted "Intifada! Intifada!", calling for a new uprising against Israel. They clashed with Israeli police in one of the most highly charged displays of enmity in Jerusalem in years.

Palestinian officials trying to calm tensions have said they would prevent any intifada, or uprising, and seek a solution to the crisis that began when the three Israeli teenagers were kidnapped.

The discovery of the young Israelis' bodies on Monday prompted an outpouring of national grief in Israel.

In a separate incident, it was claimed by relatives that Abu Khdeir's 15-year-old cousin, Tariq, a US citizen who goes to school in Florida, was beaten by police during clashes on Thursday ahead of the funeral. His parents, Suha and Salah, said Tariq was detained but had been treated at an Israeli hospital. The US State Department said it was "profoundly troubled" by the reports and demanded an urgent investigation.

Samri said Tariq had attacked police and resisted arrest. He was detained with a slingshot in his possession used to hurl stones at police, along with six other protesters, including some armed with knives, she said. Tariq's father said he witnessed his son's arrest and insisted that the boy was not involved in the violence, adding that several officers were hurt in that specific protest, one of many that day