Israel today entered the escalating Palestinian conflict between Hamas and Fatah fighters when it launched air strikes on Gaza.

Palestinian officials said an Israeli helicopter gunship had fired three missiles at Hamas buildings in the town of Rafah, leaving several people buried in the rubble.

The strikes were retaliation for Hamas militants firing rockets into Israel. The rocket attacks, which caused no reported injuries, were interpreted by some as a Hamas attempt to broaden the internal conflict and unite Palestinians against the Israelis.

In the bloodiest day in Gaza since the latest violence erupted four days ago, Hamas gunmen shot at least five Fatah bodyguards dead and mistakenly ambushed a jeep carrying their own fighters, killing five people.

The ambush happened after policemen from the Fatah-allied Preventive Security Organisation arrested five Hamas men and were driving them through Gaza City. Five Hamas men and two from Fatah were killed.

Earlier today, Hamas gunmen killed at least five Fatah guards when they attacked the Gaza home of Mr Abbas's leading security chief.

The fighting at Rashid Abu Shbak's house happened soon after mortars landed near the president's Gaza office and gunmen attacked a Hamas position outside the interior ministry building.

A Fatah official said at least six security men, including one member of the presidential guard, had been killed and at least 15 others wounded. Mr Abu Shbak was apparently not at his home at the time of the attack.

The unrelenting violence has left the two-month unity government of Hamas Islamists and the secular Fatah faction of the Palestinian president, Mahmoud Abbas, in tatters.

"What is happening in Gaza endangers not only the unity government but the Palestinian social fabric, the Palestinian cause and the Palestinian strategy as a whole," the Palestinian negotiator Saeb Erekat said.

A spokesman for the al-Aqsa Martyrs Brigades militant group, part of Fatah, blamed Hamas for the violence. "Hamas's political leadership is participating in the assassination and murder of Fatah men," Abu Qusai told Reuters.

Hamas blamed Fatah, saying the attack on Mr Abu Shbak's home had begun when shots were fired from the house at a passing Hamas patrol.

Elsewhere, gunmen pounded the main headquarters of the Fatah-dominated preventive security service with mortar bombs, sparking another gun battle in that area.

The death toll from the four days of Gaza fighting now stands at 30, with dozens more having been injured.