GAZA (Reuters) - Gaza’s ruling Hamas movement executed three Palestinians on Thursday convicted of collaborating with Israel, hanging the men during a campaign to persuade any Israeli-recruited agents to come forward in return for more lenient punishment.

Hamas leaders and officials, along with the heads of Gaza clans and local security chiefs, watched the execution in the courtyard of the enclave’s main police headquarters.

The Hamas-run Interior Ministry said the men, aged 32, 42 and 55, were convicted of treason and spying for hostile foreign parties and had provided information that helped Israel track and kill Palestinians, including leaders of militant factions.

Their trials were conducted by Hamas-run security courts in the past two years and they were put to death after all legal procedures were exhausted, the ministry said.

Since seizing control of Gaza from Palestinian President Mahmoud Abbas’s Fatah party in 2007, Hamas-run courts have sentenced 106 people to death and executed 22 of them, Palestinian and international rights groups say.

New York-based Human Rights Watch condemned Thursday’s hangings.

“Hamas authorities will never achieve true security or stability through firing squads or by the gallows, but rather through respect for international norms and the rule of law.” Sarah Leah Whitson, executive director of the group’s Middle East division, said in a statement.

Hamas issued a public ultimatum for “collaborators” to turn themselves in after the killing last month of Mazen Fuqaha, a senior leader of its armed wing, accusing Israel or its local agents of assassinating him.

Punishment would be softened for those who surrender and they would remain anonymous, Hamas said, holding out the prospect of protecting their families from the stigma attached in Gaza to cooperating with Israel.

Hamas, which has arrested an unknown number of suspects in the investigation of Fuqaha’s death, has not said if anyone has taken up the offer.

Suggesting that Fuqaha was assassinated as a result of internal rivalries, Israeli Defence Minister Avigdor Lieberman said in public remarks that Hamas should look at its own people to find the killers. Hamas officials said Lieberman was attempting to obscure Israel’s role in the death.

Over the years, Israel has established a network of contacts in the Palestinian territories, using a combination of pressure and sweeteners to entice Palestinians to divulge intelligence.

Palestinian and International Human Rights groups have repeatedly condemned death penalty, and urged Hamas and the Palestinian Authority to suspend it. Palestinian law says President Mahmoud Abbas, who has no actual control over Gaza, has the final word on whether executions can be carried out.