Israel's prime minister and defence chief will be called to testify in an investigation into a deadly raid on a Gaza aid flotilla, the leader of an Israeli commission of inquiry said in an opening statement on Monday.

An international observer on the commission said everyone involved was determined it would be rigorous.

Turkey, angered by the killing by Israeli commandos of nine Turkish pro-Palestinian activists in a melee on a Gaza blockade-running vessel intercepted on May 31, has said the Israeli probe would be biased.

Rejecting U.N. Secretary General Ban Ki-moon's proposal for an international inquiry, Israel's cabinet set up the commission, headed by retired Supreme Court Justice Jacob Turkel, two weeks ago.

The panel includes two other Israelis -- an international law expert and a former general -- and two non-voting foreign observers: David Trimble, a Northern Ireland politician and Nobel Peace Prize laureate, and Canadian jurist Ken Watkin.

"The commission has decided to summon soon the prime minister, the defence minister and the chief of staff, along with other senior officials, as it deems fit," Turkel said in opening remarks before the session was closed to the media.

Trimble said everyone on the panel was "determined the inquiry be rigorous and hope that it can, thereby, make a positive contribution to peace".

Prime Minister Benjamin Netanyahu has said he would be prepared to testify, along with Defence Minister Ehud Barak and Lieutenant-General Gabi Ashkenazi, Israel's top general.

LIMITED MANDATE

Turkel said the panel's mandate calls for an examination of whether Israel's naval blockade and the flotilla's interception conformed with international law and also investigate the actions taken by the convoy's organisers and participants.

Israel has said the naval blockade was necessary to prevent arms shipments from Iran to Gaza's Hamas Islamist rulers.

Although Netanyahu will face questioning by the commission, its narrow mandate meant that its findings, which Israel has said would be made public, were unlikely to pose a threat to his governing coalition.

Israeli leaders have said the troops, on boarding the Turkish-flagged Mavi Marmara ship, opened fire in self-defence after coming under attack from activists wielding metal rods and knives.

Turkey, once Israel's close strategic ally, called the bloodshed Israeli "state terrorism", withdrew its ambassador from Israel and cancelled joint military exercises.

Public opinion polls in Israel have shown wide support for the operation, amid questions over whether military planners had underestimated the type of resistance the commandos could face.

Under international pressure, Israel eased its Gaza land blockade on June 20, saying it would now allow all goods, except weapons and material that could be used to make them, into the territory.

The naval blockade of the enclave, home to 1.5 million Palestinians, remains in effect.