Hamas said on Thursday it executed by firing squad two Palestinians convicted by a Gaza military court in 2009 of collaborating with Israel.

It was the first time that Islamist group has carried out formal executions since the it seized control of the Gaza Strip three years ago from the rival Fatah party of President Mahmoud Abbas, which governs in the occupied West Bank.

A Hamas government official confirmed the executions after the bodies of two men arrived at a Gaza hospital. The group's interior ministry said they were carried out by firing squad.

The Palestinian rights group al-Mizan identified the executed men as Mohammed Ismail and Nasser Abu Freh.

Al-Mezan and four other rights groups condemned the execution and said it would not deter others.

"It is inhumane and humiliating and cannot achieve any rehabilitation and there is no chance for those punished to regain a place in the community," said a statement issued by five rights groups and a leading mental health institution.

The groups also said Hamas had violated the law by carrying out the executions without Abbas's authorisation and they urged the Islamist faction to desist from further executions. A member of one of the rights groups who contacted the families of the two dead men said they had been allowed to see their relatives last night but had not been told of the intention to execute them.

He said that the families were notified early on Thursday that the bodies had been taken to Gaza's main hospital.

Under Palestinian law, execution orders can be carried out only with presidential approval. But Hamas does not recognise Abbas's presidency, and the executions represented a further challenge by the group to his authority.

Abbas has refrained from approving death sentences against Palestinians convicted of capital offences in the West Bank.

London-based Amnesty International had urged Hamas not to carry out several death sentences against Palestinians convicted of murder and of collaborating with Israel.

Amnesty said proceedings in Hamas military courts do not meet international legal standards. It added that the last execution in Gaza took place in 2005