Israeli aircraft struck Hamas security compounds across Gaza today, killing at least 225 people, according to medical chiefs.

Hamas and medics reported that more than 700 others were hurt with some still buried under the rubble.

In one Hamas compound, bodies of more than a dozen uniformed security officers were seen lying on the ground.

Witnesses have claimed that mong the dead was the Gaza police chief, Maj. Gen. Tawfiq Jaber.

In a statement, Israel confirmed it carried out a series of air strikes on Hamas installations, but did not provide further details.

Israeli Defence Minister Ehud Barak said "the operation will last as long as necessary," but it was not clear if it would be coupled with a ground offensive. Asked if Hamas political leaders might be targeted next, military spokeswoman Major Avital Leibovich said, "Any Hamas target is a target."

Mr Barak said that the coming period "won't be easy and won't be short for the communities in the south (of Israel)".

The strikes - which targeted more than 100 security compounds and rocket-launching pads - caused widespread panic and confusion in Gaza, ruled by Islamic Hamas militants for the past 18 months. Some of the Israeli missiles struck in densely populated areas as children were leaving school, and women rushed into the streets frantically looking for their children.

The first round of air strikes came just before noon, and several more waves followed.

Israel has warned in recent days it would strike back hard against continued rocket fire from Gaza on Israeli border towns.

Defiant Hamas leaders threatened revenge, including suicide attacks. Hamas "will continue the resistance until the last drop of blood," said spokesman Fawzi Barhoum.

One man sat in the middle of a Gaza City street, close to a security compound, alternately slapping his face and covering his head with dust from the bombed-out building.

"My son is gone, my son is gone," wailed Sadi Masri, 57. The shopkeeper said he sent his son out to buy cigarettes minutes before the airstrikes began and now could not find him. "May I burn like the cigarettes, may Israel burn," Masri moaned.

Civilians rushed to the targeted areas, trying to move the wounded in their cars to hospitals.

In the West Bank, Hamas' rival, moderate Palestinian President Mahmoud Abbas, said in a statement that he "condemns this aggression" and called for restraint.

A White House spokesman said the United States "urges Israel to avoid civilian casualties as it targets Hamas in Gaza".

"Hamas' continued rocket attacks into Israel must cease if the violence is to stop," the spokesman, Gordon Johndroe, added.

Israel has targeted Gaza in the past, but the number of simultaneous attacks was unprecedented.

Israel left Gaza in 2005 after a 38-year occupation, but the withdrawal did not lead to better relations with Palestinians in the territory as Israeli officials had hoped.

Instead, the evacuation was followed by a sharp rise in militant attacks on Israeli border communities that on several occasions provoked harsh Israeli military reprisals.

The last, in late February and early March of this year, spurred both sides to agree to a truce that was to have lasted six months but began unravelling in early November.