GAZA, April 15 (Reuters) - Hamas said on Thursday it executed by firing squad two Palestinians convicted by a Gaza military court of collaborating with Israel.

It was the first time that Hamas has carried out formal executions since the Islamist group seized control of the Gaza Strip three years ago from the rival Fatah party of President Mahmoud Abbas, who 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.

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.

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.

Amnesty said the last execution in Gaza took place in 2005. (Reporting by Nidal al-Mughrabi; Writing by Joseph Nasr; Editing by Jon Hemming)