The Islamist Palestinian group Hamas said on Wednesday it had begun reasserting control in the Gaza Strip and rounding up suspected collaborators with Israel, drawing accusations from the rival Fatah group that its members were being targeted.

"The internal security service was instructed to track collaborators and hit them hard," said Ehab al-Ghsain, spokesman of the Hamas Interior Ministry, without singling out Fatah members by name.

"They arrested dozens of collaborators who attempted to strike the resistance by giving information to the occupation about the fighters," he said, using a Hamas term for Israel, whose 22-day offensive devastated the Gaza Strip.

Earlier this month, Haaretz correspondent Amira Hass first reported the secret executions of dozens of Gazans consider by Hamas to be "collaborators" with the enemy.

Hamas and Fatah supporters have traded accusations in Arab media that Fatah collaborated with Israel in the hope it could return to power in the Gaza Strip and that Hamas provoked the Israeli invasion by firing rockets into southern Israel.

Hamas, an Islamist group that won the 2006 Palestinian election, violently seized the coastal enclave from Fatah, led by Palestinian President Mahmoud Abbas, in 2007.

A statement issued by Fatah in Gaza said that since fighting between Israel and Gaza militants ended - Hamas and Israel put separate ceasefires into effect on Sunday - Hamas militias had carried out a number of attacks against Fatah members.

These attacks, according to the statement, included "shooting at the feet of Fatah members, brutal crimes of execution and throwing the bodies in the rubble of destruction." Fatah appealed to Abbas' Palestinian Authority to intervene.

Ghsain denied the allegations but said authorities had begun tracking down suspected "collaborators" with Israel.

"These are familiar lies and false allegations from Fatah and the forces of sedition," he said.

"It is also an attempt to steal the light of victory and cover up how they bet on the enemy and their disappointment that the enemy could do nothing to return them [to power]."

Residents said several suspected collaborators were killed by unknown assailants during the Gaza war. No Hamas-related groups have claimed responsibility and relatives of militants killed by Israel are thought to be behind some of the attacks.

Some prisoners escaped from the Israeli-bombed security complex housing Gaza's main prison early in the conflict. At least two were subsequently shot in a settling of scores with people suspected of collaboration with Israel.

The offensive Israel launched on Dec. 27 with the declared aim of ending rocket attacks from the Gaza Strip killed nearly 1,300 Palestinians and made thousands homeless. Gaza medical officials said the Palestinian dead included at least 700 civilians.

Israel, which put its own losses at 10 soldiers and three civilians, says hundreds of militants died. It completed a Gaza troop pullout on Wednesday.

The Hamas government said many employees were back at work on Wednesday in buildings such as the Ministry of Education that survived the Israeli attack intact. Employees of destroyed ministries would be relocated to other buildings.

Armed police in official uniforms were on the streets, organizing traffic and guarding government buildings. Hamas says 230 policemen were killed during the fighting. An Israeli attack also killed Hamas' security chief, Said Sayyam.