Correspondents say the move - in addition to the call-up of thousands of reservists - could be a prelude to ground operations, but could also be intended to build pressure on Hamas.
BBC analyst Jonathan Marcus says the 6,500 reservists called up would be insufficient for a knockout blow against Hamas, which would require nothing less than the re-occupation of the Gaza Strip.
Dozens of centres of Hamas strength, including security compounds, government offices and tunnels into Egypt, have been hit since Israel started its massive bombing campaign on Saturday morning.
As dawn broke on Monday, witnesses said a powerful explosion struck the interior ministry.
Earlier, a raid destroyed a science building at the Islamic University in Gaza. Israel has claimed that facilities in the building have been used for weapons production.
Also hit was a house near the abandoned home of a senior Hamas leader in the southern town of Rafah.
The UN relief agency in Gaza says 57 civilians have been killed by Israeli fire so far, the latest being five girls who died in Jabaliya refugee camp when Israeli forces bombed a mosque near their home.
A small number of wounded Palestinians have begun passing through the Rafah crossing into Egypt. About 30 were expected to leave for treatment in the course of Monday, border officials said.
The Israeli action has sparked anger across the Arab world.
In the West Bank, a Palestinian reportedly stabbed at least three Israelis in a Jewish settlement before he was shot and arrested.
Protests have been held in countries including Syria, Jordan, Iraq and Lebanon, as well as in a number of European capitals.
At the UN, the Security Council joined international calls for restraint by urging an end to all violence between Israel and Gaza.
The US, Israel's strongest ally on the council, said the onus was on Hamas to stop rocket fire first and commit itself to a truce.
Israel says Palestinian militants have fired more than 110 rockets from the coastal territory since Saturday.
RUN-UP TO GAZA RAIDS
Shaky Egyptian-brokered six-month truce between Hamas and Israel agreed on 19 June
Palestinian rocket fire and Israeli operations in Gaza reduced
Violence resumes on 4 Nov as Israel launches incursion which it says targets Hamas tunnel; Palestinians respond with rocket fire
Hostilities increase; Israel tightens blockade on Gaza
Truce expires on 19 Dec; both sides blame each other for its breakdown; rocket fire increases
Israel launches major air strikes on 27 Dec
The strikes began on Saturday less than a week after the expiry of a six-month-long ceasefire deal with Hamas.
Ehud Barak told a special parliamentary session that Israel was "taking all precautions" to avoid harming Palestinian civilians, but blamed militants for intentionally hiding in the civilian population.
His statement, and that of Foreign Minister Tzipi Livni on Sunday about "changing realities on the ground" in Gaza, have fuelled speculation about a sustained Israeli bombing campaign.
Analysts said Saturday was the single deadliest day in Gaza since Israel's occupation of the territory in 1967, although no independent confirmation is available of the numbers killed.
The violence began days after a six-month truce expired between Israel and Hamas, and as Israel was preparing for a general election in February.
The exiled leader of Hamas, Khaled Meshaal, has called for a new intifada, or uprising, against Israel, while the movement's Gaza leader, Ismail Haniya, called the attack an "ugly massacre".
Attacks over the Gaza-Israel border regularly put the truce under strain with both sides blaming each other for unprovoked violations.
From 1967, Israel's military occupied the Gaza Strip and Jewish settlers built communities within the territory. Israel withdrew in 2005 but has kept tight control over access in and out of Gaza and its airspace.
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