Tony Abu Aqla told the BBC the findings were "unacceptable" and insisted his sister was targeted by Israeli troops.

The Al Jazeera correspondent was shot in the head while covering an Israeli raid in the occupied West Bank in May.

The US said the bullet was too damaged to tell where it had been fired from.

A top official in the Palestinian Authority, which has concluded that Abu Aqla was intentionally shot dead by an Israeli soldier, accused the US of trying to protect Israel.

Israel's prime minister expressed sorrow over Abu Aqla's "tragic" death and said an investigation by its own military had determined "conclusively that there was no intention to harm her".

Shireen Abu Aqla, who was 51, was one of the Middle East's most experienced and well-loved correspondents.

She was wearing a flak jacket marked with the word "Press" as well as a helmet when she was killed while walking down a road in the West Bank city of Jenin on 11 May, near where a gun battle between Israeli forces and Palestinian militants had taken place. Another Palestinian journalist, Ali Samoudi, was wounded.

Samoudi and other journalists with them said the gunfire came from Israeli troops stationed on the road. Several investigations by the media and one by the UN Human Rights Office also concluded that Israeli forces fired, or seemingly fired, the fatal shot.

On Monday, the US State Department said independent examiners had carried out a forensic analysis of the bullet - which was handed over by the Palestinian Authority to the US Security Co-ordinator (USSC) at the weekend - but that it was too badly damaged to allow them to work out whether an Israeli or Palestinian gun fired it.

However, after reviewing the evidence from both sides, the USSC "concluded that gunfire from IDF [Israel Defense Forces] positions was likely responsible for the death of Shireen Abu Aqla", the statement said. "The USSC found no reason to believe that this was intentional but rather the result of tragic circumstances," it added.

In an interview with the BBC's Newshour program, Tony Abu Aqla said: "We cannot really believe that the US state department would come up with such a statement. It's unacceptable."

He added: "It was obvious that there were 16 bullets fired, all at the same head level, and one I think 1.3m [4.3ft] high, which is targeting the head. Only journalists were hit, two journalists. Shireen unfortunately didn't make it. Ali Samoudi was injured... I would like to know how they come up with that it wasn't deliberate."

Abu Aqla said his family would do everything possible to hold those responsible to account if US authorities would not act.

"They just can't walk away from this. We have to pursue justice for Shireen, and if the US are not able to pursue justice for a US citizen then it's too bad."

Senior Palestinian Authority official Hussain al-Sheikh said it would "not allow attempts to conceal the truth or to have shy references in pointing the finger of accusation to Israel" and promised to continue pursuing the case through the international courts, where it has been presented as a possible war crime.

Israeli Prime Minister Yair Lapid said: "The IDF will continue fighting terror whenever and wherever necessary.

"The IDF investigation was unable to determine who is responsible for the tragic death of journalist Shireen Abu Aqla, but it was able to determine conclusively that there was no intention to harm her. Israel expresses sorrow over her death," he added.