(Adds Blair comments, paragraph 3)

By Nidal al-Mughrabi

GAZA, June 27 (Reuters) - Israeli forces killed at least 12 Palestinians, mostly gunmen but also a 12-year-old boy and other civilians, on Wednesday in the deadliest raid in the Gaza Strip since Hamas seized the territory, medical workers said.

The violence erupted as Middle East mediators prepared to name Tony Blair, who stepped down as Britain's prime minister, as their new envoy in a bid to revive Israeli-Palestinian peacemaking after the Islamist militant group's Gaza victory.

"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," Blair told parliament in London.

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 (Palestinian President Mahmoud) 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, which routed forces from Abbas's secular Fatah faction in the territory two weeks ago.

Local residents in two Gaza battle zones said gunmen fired rocket-propelled grenades and detonated explosive devices in confrontations with Israeli infantry and armour.

BOY KILLED

A 12-year-old lay in the street, his arms twisted at odd angles, near a house in a Gaza City neighbourhood 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" to foil rocket attacks from Gaza.

Commenting on the raid, Abbas told reporters: "We strongly condemn these criminal acts, either in Gaza or the West Bank. We are against violence in all its forms and also we are against launching rockets (at Israel)."

At a news conference with Abbas in the West Bank, Russian Foreign Minister Sergei Lavrov said the Quartet of Middle East mediators -- the United States, the European Union, the United Nations and Russia -- had agreed on a mandate for a new envoy.

Ghazi Hamad, an aide to Ismail Haniyeh, prime minister in the Hamas-led government dismissed by Abbas, said the movement did not expect Blair to be even-handed.

"He has always adopted the American and Israeli positions," Hamad said.

Hamas, which came to power in a 2006 election, has rejected Western demands to recognise Israel, renounce violence and accept existing interim Israeli-Palestinian peace deals.

Israel pulled troops and settlers out of Gaza in 2005 but has not stopped air strikes and other attacks against militant groups that frequently fire rockets into southern Israel. (Additional reporting by Jeffrey Heller in Jerusalem and Haitham Tamimi in Ramallah)

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