GAZA, Jan 29 (Reuters) - A Palestinian man on Thursday accused Islamist Hamas militants in control of the Gaza Strip of torturing and killing his brother for publicly criticising them.

Osama Atallah, a teacher, was a supporter of the Fatah movement of Palestinian President Mahmoud Abbas, the sworn enemy of Hamas, whose gunmen drove Fatah militia out of Gaza in 2007 and fought Israel's army in a three-week war this month.

His brother Bassam said masked gunmen in two jeeps arrived at the family home in the city of Gaza on Tuesday. They identified themselves as members of Hamas internal security and they arrested his brother Osama.

Bassam said the Hamas security service told the Atallah family Osama would be released in a matter of hours. But a Hamas government official, who is also a member of the Atallah family, later denied the teacher was in custody.

The family subsequently received a telephone call from hospital that Osama Atallah was in critical condition.

He later died of his wounds.

Ten Israeli soldiers died in the 22-day offensive, including four in a "friendly fire" accident. About 1,300 Palestinians were killed, according to a Gaza human rights group, of whom over 700 were civilians.

Many older Gaza Palestinians scoffed at a claim of "victory" by Hamas once the fighting was halted on Jan 18.

Fatah sources said Osama Atallah, a Fatah activist, had been threatened by Hamas "because of his public and continued criticism of the performance of the Hamas militias in Gaza."

They accused Hamas of "severely torturing and then strangling" Osama Atallah. They said bullets in his body could have been fired after he died.

Hamas interior ministry spokesman Ehab al-Ghsain did not respond to telephone calls from from Reuters about the accusation.

Hamas security officials said "dozens of collaborators" had been detained, including people they accused of spying for Israel during this month's battles.

The Israeli daily Haaretz reported on Thursday that Hamas "executed several dozen civilians" during and after Israel's assault on Gaza. Some were members of Fatah, but others were not politically affiliated, the paper said.

Haaretz quoted an Israeli intelligence source as saying a number of Palestinian agents working for Israel were intercepted by Hamas "because the intelligence they provided was used carelessly" by commanders intent on minimising troop casualties.

"It appears that in most cases Hamas suspected that their victims had collaborated with Israeli intelligence," it said.

Hamas in Gaza has not confirmed killing collaborators. But an Arab newspaper has carried a statement by an exiled Hamas official confirming that the group killed several.

The accuracy of some Israeli airstrikes targeting individual militants in buildings, or trying to snipe at or pinpoint Israeli forces, certainly came from spies on the ground, according to some Palestinian security sources.

Haaretz said Israeli commanders with troops in Gaza were "very impressed that (intelligence services) could warn them with great precision of developments in their proximity".