"Extrajudicial killings have increased during the Israeli aggression."
Separately on Sunday, the Gaza-based Palestinian Center for Human Rights called for an investigation into the death of a man beaten in the custody of security forces loyal to the Hamas movement.
Jamil Shakoura died in a Gaza hospital after receiving a number wounds to his head while detained, the group said.
He was not believed to be affiliated to any political group and it was not clear why the security forces were investigating him.
Human rights groups say that at least three people have died while in detention since Hamas seized full control of the Gaza Strip in June 2007 after pushing out security forces loyal to Mahmoud Abbas, the Palestinian president and head of Fatah.Abuse allegations
Khalil Abu Shammala, a human rights monitor, told Al Jazeera that dozens of Fatah members were shot and tortured as Israeli forces bombarded the Gaza Strip for 22 days.
"I don't ignore that there were some mistakes made my members of Hamas, but I think that is not a policy adopted by the leadership here"
Naem Atallah told Al Jazeera that he found the body of his son Osama, a Fatah supporter, at Gaza's Shifa hospital after he was taken away from the family home by 10 masked men.
"I asked them who are you? They answered that they are from the internal security. I asked how do I know you are from internal security and one showed me his ID card," he said.
Osama, a teacher and father of five children, had been strangled, suffered blunt force trauma to his head and been shot in the shoulder.
Another Fatah supporter, who asked to remain anonymous for fear of reprisals, said that that he had been shot in the leg three times by Hamas loyalists.
"They shot me because I am Fatah," he told Al Jazeera.
"There were 15 guys with hoods over their heads. Three of them asked me to stand against the wall and asked me my name. Three started shooting."
Hamas officials have said that they have several investigations under way into the claims of extrajudicial beatings, torture and killings.
"All our investigations are open to everyone," Ihab al-Ghusain, a spokesman for the Hamas interior ministry, told Al Jazeera.
"Nobody is allowed to kill anyone, beat anyone, arrest anyone."
However, all of Al Jazeera's attempts to arrange to view the files on the ongoing investigations went unanswered.
Relations between Hamas and Abbas's Fatah have been strained since 2007, but Ghazi Hamad, a senior Hamas official, denied that the movement's leaders are encouraging a policy of targeting rival Palestinians.
"I don't ignore that there were some mistakes made by members of Hamas, but I think that is not a policy adopted by the leadership here," he said.
"I think it's a shame for Palestinians to arrest each other or torture each other; its very shameful so we have to stop it."