Israel turned the force of its brutal Gaza campaign towards Hamas leaders and agents yesterday, sending warplanes to bomb their houses in a sweep meant to tear at the roots of the militant organisation.

Israel turned the force of its brutal Gaza campaign towards Hamas leaders and agents yesterday, sending warplanes to bomb their houses in a sweep meant to tear at the roots of the militant organisation.

As Hamas sites were reduced to rubble and Israel continued to strike from the skies with its fleet of warplanes, helicopters and pilotless drones, the targets Israel had chosen revealed an intention to chip away at Hamas's foundation.

Israel carried out five separate strikes on the houses of field operatives.

And in grainy surveillance video from an overhead drone released by the Israeli military, several men are seen loading a truck with what the military said were medium-range Grad rockets. Moments later, a large explosion from a missile strike envelops the image.

The Israeli army believed Hamas was transferring the missiles to a hide-out for fear their location had been compromised.

One of the strikes targeted the home of Maher Zakut, the commander of Hamas's rocket-firing forces, the army said. It was unclear whether Zakut was in the house. Palestinian sources said seven people were killed in the strike.

Earlier, Palestinian sources reported that Ziad Abu-Tir, 36, a senior activist, had been killed along with four others, in an air strike in the central Gaza Strip, near Khan Yunis.

Another target was the Jabaliya home of Abdel-Karim Jaber, a Hamas political figure. Jaber, a senior administrator at Gaza's Islamic University, was not at home and it was not clear if anyone was hurt in the strike.

In another air assault, an Islamic Jihad commander was killed walking near his home.

Aircraft also flattened a house next to the home of Hamas premier Ismail Haniyeh and a security compound linked to the Islamic group.

In the early hours yesterday, in what Israelis described as a strategic blow to the Hamas military wing, two research and development laboratories on the Islamic University campus in Gaza City were bombed.

Witnesses said they saw fire and smoke billowing at the university, counting six separate air strikes there just after midnight.

As some Gazans inspected the damage, many others stayed at home, seeking shelter from the onslaught.

Israeli officials said the explosives developed and manufactured in the laboratories were used to make sophisticated explosive devices and mortars used against Israel. Many Hamas officials graduated from the university.

Other targets included a guest palace used by the Hamas government.

Air strikes on more than 320 sites since Saturday had already reduced dozens of buildings to rubble, overwhelmed hospitals with wounded and filled Gaza's deserted streets with smoke and fire. The military said naval vessels had also bombarded targets from the sea.

For the first time, Israel also hit one of a series of tunnels Hamas prepared along the border with Israel for use in attacks on invading ground forces, Israeli TV reported. One tunnel was packed with explosives and several militants inside were killed.

Most of those killed in three days of air strikes have been Hamas members, but the UN agency in charge of Palestinian refugees said at least 51 of the dead were civilians. A rise in such casualties could intensify international pressure on Israel to abort the offensive.

A Hamas police spokesman, Ehab Ghussen, said 180 members of the Hamas security forces were among the dead.

Still, militant rocket barrages continued. Sirens warning of incoming rockets sent Israelis scrambling for cover yesterday as more than 40 rockets and mortar rounds rained down.

One medium-range rocket fired at the Israeli city of Ashkelon killed an Arab construction worker and wounded several others. He was the second Israeli killed in the offensive and the first person killed by a rocket in Ashkelon, a city of 120,000.

In Gaza, families left apartments next to institutions linked to Hamas, fearing they could be targeted.

Gaza City centre was plunged into darkness after its power station stopped functioning because Israel blocked deliveries of diesel oil for the generators.

Suad Abu Wadi, 42, was keeping her six children close on mattresses in her Gaza City living room. Her husband sat with them, chain-smoking.

She said he had not spoken a word since seeing their neighbour carrying the body of his child, killed in an air strike on Saturday.

Gaza's nine hospitals were overwhelmed. Dr Moaiya Hassanain, who keeps a record for the Gaza Health Ministry, believed 364 Palestinians had died and more than 1400 had been wounded. Some of the injured are now taken to private clinics and even homes, he said.

Israeli security officials warned the militants' range now includes Beersheba, a city 30 miles from Gaza Resident Mazal Ivgi, 62, said she had prepared a bomb shelter. "In the meantime we don't really believe it's going to happen, but when the first boom comes people will be worried," she said.

An Israeli TV graphic illustrated the rocket reach with colour-coded circles. Those in closest range were advised to stay within a 15-second dash of a bomb shelter.