International powers named Tony Blair their Middle East peace envoy yesterday, handing the outgoing British prime minister a daunting new challenge on a day Israeli forces killed 12 Palestinians in the Gaza Strip.

The Quartet of would-be peacemakers - the US, UN, EU and Russia - made the expected announcement shortly after Mr Blair handed power to party colleague Gordon Brown. Last-minute Russian hesitation was overcome. Mr Blair, who led Britain for 10 years, has strong backing from US President George W. Bush. Israel has said it approves of the choice as have aides to Palestinian President Mahmoud Abbas.

But the Hamas Islamists who routed President Abbas's forces in Gaza two weeks ago, complicating efforts to found a Palestinian state in both the coastal enclave and the inland West Bank, derided Mr Blair as pro-Israeli. Arab states also have reservations about a man identified with backing for Mr Bush's invasion of Iraq.

Tony Blair himself told parliament in his final appearance:

"The absolute priority is to try to give effect to what is now the consensus across the international community - that the only way of bringing stability and peace to the Middle East is a two-state solution... I believe it is possible to do that but it will require a huge intensity of focus and work."

The Quartet expects Mr Blair to work with President Abbas on building up institutions needed for a future state, diplomats said. With Gaza under Hamas control and effectively cut off from Mr Abbas's secular, Fatah-dominated West Bank, that is a major challenge.

Mr Blair will "mobilise international assistance for the Palestinians", UN spokesman Michele Montas said, adding that he had "long demonstrated his commitment on these issues".

Israeli forces killed at least 12 Palestinians, mostly gunmen but also a 12-year-old boy and other civilians, yesterday in the deadliest raid in the Gaza Strip since Hamas seized the territory, medical workers said.

Israel's operation in Gaza City and the southern town of Khan Younis appeared to signal it intended to keep strong military pressure on Hamas along with efforts to isolate the movement financially and politically.

Hamas spokesman Fawzi Barhoum said the Israeli incursion was part of a "conspiracy in which Abbas is a participant and which is aimed at pressuring Hamas and the people of Gaza".

Four of the nine militants killed in the Israeli operation belonged to Hamas. Residents in two Gaza battle zones said gunmen fired rocket-propelled grenades and detonated explosive devices in confrontations with Israeli infantry and armour.

A 12-year-old lay in the street, his arms twisted at odd angles, near a house in Gaza City where residents and medical workers said a shell fired by an Israeli tank exploded. He was pronounced dead in a hospital along with two men, their bodies shredded by shrapnel.

Residents said the men were civilians.

A military spokesman in Tel Aviv said a tank shell fired in Gaza City's Shejaia neighbourhood was aimed at a gunman, and he had no information about a house being hit. Residents said tanks in the area later withdrew towards the Israeli frontier.

Two Israeli soldiers were wounded by an anti-tank missile during operations that Israel's deputy defence minister, Ephraim Sneh, described as "preventive measures" against rocket attacks. Several rockets fired from Gaza hit Israel, hurting no one.

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