Islamist militants from Hamas committed war crimes against fellow Palestinians in Gaza – including killings, kidnappings and torture – during the summer 2014 conflict with Israel, Amnesty International said in a report released Wednesday.

The London-based rights group detailed a "brutal campaign of abductions, torture and unlawful killings against Palestinians accused of 'collaborating' with Israel" by Hamas, which controls the Gaza Strip.

"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," said Amnesty's Middle East and North Africa director, Philip Luther.

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

The report, entitled "Strangling Necks", 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.

He accused Hamas of "appalling crimes against powerless individuals" that in some cases constituted "war crimes".

A Hamas spokesman rejected the human rights 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.

Targeting Fatah

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”, the Amnesty report said.

"Not a single person" has been held accountable for crimes committed by Hamas, which the report said indicated "that these crimes were either ordered or condoned by the authorities".

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

Around 2,200 Palestinians were killed during last year's 50-day conflict with Israel, which also left 73 dead on the Israeli side as Israel and Hamas engaged in tit-for-tat cross-border rocket and mortar attacks.

A March report by Amnesty International found that rockets fired by Hamas and other Gaza-based militants killed more Palestinians than Israelis.

Hamas has been accused of purposefully locating its rocket launchers in residential areas to discourage Israel from targeting them – or to ensure that Israel comes under fire for causing civilian casualties if it does.

Amnesty also accused Israel of committing war crimes during its Gaza campaign in a December report. Israel's military has opened its own investigation into incidents including an Israeli air strike on a Gaza beach that killed four boys and tank fire on a UN school in the town of Beit Hanoun.

Palestinian officials are widely thought to be preparing to sue Israel through the International Criminal Court for alleged war crimes, having officially joined the ICC on April 1.

A Gaza ceasefire agreed in August between Israel and Palestinian factions called for the Palestinian Authority, led by Mahmoud Abbas, to take over the civil administration of Gaza from Hamas. But factional infighting has left Hamas the de facto rulers of the enclave.

(FRANCE 24 with AFP)