Israeli air raids are continuing over Gaza for a fourth day, with Hamas government buildings and security installations the principal targets.
Prime minister Ehud Olmert says the government is giving the military its full backing and room to manoeuvre.
Another top official said Israel was ready for "long weeks of action".
Deputy defence minister Matan Vilnai says the military "has made preparations for long weeks of action."
Mr Olmert's statement that the bombardment was "the first of several stages approved by the security cabinet" was quoted from a briefing he gave to President Shimon Peres on Tuesday.
Opposition leader Binyamin Netanyahu has backed the offensive, telling the BBC that "Israel is using a fraction of its power to try to target surgically the terrorists".
Israel says the bombing is in relation for continuing rocket attacks.
Hamas ended a six-month truce with Israel on 19 December and the bombings began on Saturday morning, 27 December.
Border build-up
The BBC reports Israeli tanks and troops are massed along Gaza's border.
Correspondents say this could be a prelude to ground operations, but could also be intended to build pressure on Hamas, which has controlled Gaza since 2007.
Israel pulled out of Gaza in 2005 but has kept tight control over access in and out of the narrow coastal strip and its airspace.
A general election is to be held in Israel on 10 February.
