(Adds Israeli threat, adds details on casualties)

By Alastair Macdonald

NAHAL OZ, Israel, April 9 (Reuters) - Palestinian gunmen on Wednesday attacked an Israeli-controlled border crossing where fuel is piped into the Gaza Strip, killing two Israeli civilians, the Israeli army said.

Repelling the attack on the Nahal Oz terminal, Israeli troops backed by tanks killed two Palestinian militants and two civilians just within the Gaza frontier, Palestinian medics said. Another gunman died in an air strike near Gaza City.

Hours earlier, an Israeli soldier and a Hamas militant were killed in clashes in southern Gaza. Wednesday's death toll was the highest for a single day since Israeli forces waged a Gaza offensive that killed scores of Palestinians in early March. Hamas warned on Tuesday of an "explosion" in the territory if Israel continued its economic blockade. Israel tightened the cordon after the Islamist group, which is formally committed to the Jewish state's destruction, seized control of Gaza in June.

Fighters from three factions -- not including Hamas -- infiltrated the terminal, said a spokesman, describing the attack as a an attempt to abduct soldiers.

"Terrorists entered Nahal Oz and the fuel depot. They shot at civilians inside. There are two Israelis dead and two wounded," an Israeli army spokeswoman said.

Hamas said it pounded the area with mortar bombs and machine guns during the operation, which one of the groups involved, the Popular Resistance Committees (PRC), dubbed "Breaking Zionist Arrogance".

In addition to the PRC, Islamic Jihad and the Mujahideen Brigades, which is affiliated with President Mahmoud Abbas's Fatah faction, claimed responsibility for raiding Nahal Oz.

"The target was a Zionist army base and the aim was to abduct soldiers to swap them for Palestinian prisoners," said the PRC spokesman, Abu Mujahed.

ISRAEL BLAMES HAMAS

An Israeli military official at Nahal Oz, Colonel Nir Press, accused Hamas of trying to worsen Palestinian privation in Gaza.

"Today they attacked the lifeline of the energy supplies -- the lifeline of the Palestinians," he told reporters.

Israeli Foreign Minister Tzipi Livni threatened Hamas with unspecified reprisals. "Hamas bears responsibility ... with everything that implies," she said in a statement.

All fuel pumped into Gaza passes through Nahal Oz. Western diplomats said the attack occurred after completion of the latest delivery of EU-funded fuel to Gaza's main power plant.

Blaming Palestinian cross-border rocket salvoes, Israel has cut back on some fuel supplies to Gaza as part of its sanctions against Hamas.

But Livni spokesman Arye Mekel said after Wednesday's attack that Israel would continue to provide the supplies necessary to avoid a humanitarian crisis.

A European official said the EU did not expect to be able to restart its fuel deliveries to the power plant before Sunday. The plant had enough fuel for eight to nine days.

The head of the Palestinian Fuel Suppliers' Association, Mahmoud al-Khuzundar, said reserves of benzine and diesel had been depleted, leaving only enough cooking gas for two days.

Press accused Palestinians of having failed to collect some fuel delivered at Nahal Oz so ordinary Palestinians suffered.

"They are collecting the cooking gas and diesel for the plant, but not taking the petrol and the lighter diesel for cars and trucks," he said.

Spokesmen for militant groups said attackers withdrew into the Gaza Strip, under fire from Israeli helicopters, and that two Palestinians in a car hit from the air were hurt.

The army said the vehicle was the militants' getaway car.

Two later air strikes near Gaza City killed a PRC gunman and wounded another militant and three civilians, hospital officials and Hamas security sources said. (Reporting by Nidal al-Mughrabi; Writing by Jeffrey Heller, Editing by Adam Entous and Giles Elgood)

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