Jerusalem (AFP) - Amnesty International accused the Islamist movement Hamas Wednesday of committing war crimes against fellow Palestinians to "settle scores" during last year's Gaza war with Israel, executing at least 23 people.

A report by the London-based rights group detailed the "brutal campaign of abductions, torture and unlawful killings against Palestinians accused of 'collaborating' with Israel" by Hamas, de facto ruler of the Gaza Strip enclave.

"In the chaos of the conflict, the de facto Hamas administration granted its security forces free rein to carry out horrific abuses including against people in its custody," Amnesty's Middle East and North Africa director Philip Luther said.

"These spine-chilling actions, some of which amount to war crimes, were designed to exact revenge and spread fear across the Gaza Strip."

The report details the "extrajudicial execution of at least 23 Palestinians and the arrest and torture of dozens of others".

"It is absolutely appalling that, while Israeli forces were inflicting massive death and destruction upon the people in Gaza, Hamas forces took the opportunity to ruthlessly settle scores, carrying out a series of unlawful killings and other grave abuses," Luther said.

A Hamas spokesman rejected the watchdog's findings.

The report "lacks professionalism and credibility and is deliberately exaggerated, without taking into account all sides or verifying information," Fawzi Barhum told AFP.

Around 2,200 Palestinians were killed during last year's 50-day conflict with Israel, with 73 killed on the Israeli side.

The Jewish state went to war against Hamas to stamp out cross-border rocket and mortar fire.

A March report by Amnesty found that rockets fired during the war by Gaza militants killed more Palestinians than Israelis.

According to the new Amnesty report, "Hamas forces also abducted, tortured or attacked members and supporters of Fatah, their main rival political organisation within Gaza, including former members of the Palestinian Authority security forces.

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"Not a single person has been held accountable for the crimes committed by Hamas forces against Palestinians during the 2014 conflict, indicating that these crimes were either ordered or condoned by the authorities," it said.

Luther accused Hamas of "appalling crimes against powerless individuals", which in some cases constitute war crimes.

He said the militant movement displayed "a disregard for the most fundamental rules of international humanitarian law".

Amnesty called on the Ramallah-based Palestinian Authority and Hamas to "cooperate with independent and impartial international investigative mechanisms", and to bring "suspected perpetrators" to justice.

The Palestinians are preparing to sue Israeli officials through the International Criminal Court for alleged war crimes committed during last year's Gaza conflict.

Israel's military has opened investigations into deadly incidents that took place during the war.

Amnesty accused Israel in a December report of committing war crimes in its Gaza campaign.