Palestinian health officials have reported the death toll from Israeli air strikes on Hamas targets in Gaza has risen to 225.
Israel says the offensive was in response to rocket salvoes by Gaza militants that intensified after Hamas ended a six-month-old ceasefire a week ago.
Both sides said they were ready to stage wider assaults.
'It may take time, and each and every one of us must be patient so we can complete the mission,' Israeli Prime Minister Ehud Olmert told reporters after the air strikes.
A senior Israeli military source said the strikes marked the start of a campaign that could be expanded to include ground forces.
Israeli analysts said the assault could be aimed at forcing Hamas to accept tougher terms for renewing the truce.
Black smoke billowed over Gaza City, where the dead and wounded lay scattered on the ground after the air strikes destroyed more than 30 security compounds, including two where Hamas was hosting graduation ceremonies for new recruits.
Hamas called the assault a 'massacre’.
Israel said it had targeted ‘terrorist infrastructure’ following days of rocket attacks from Gaza on southern Israel that caused some damage but few injuries.
‘There is a time for calm and a time for fighting, and now the time has come to fight,’ Defence Minister Ehud Barak said.
The rocket attacks had increased pressure on Israeli political leaders to strike Hamas ahead of a 10 February election.
Mr Barak said that the military campaign would take time and would be expanded ‘as necessary’.
Hamas leaders could be targeted, an Israeli army spokeswoman said.
The mayor of Ashkelon, the Mediterranean coastal city in range of Hamas's Grad rockets, said Israeli military planners had told him the operation would last for ‘more than a week’.
Foreign Minister Tzipi Livni, a leading candidate to become Israel's next prime minister, called for international support against ‘an extremist Islamist organisation ... that is being supported by Iran’.
UN Secretary-General calls for halt to violence
UN Secretary-General Ban Ki-moon has called for an ‘immediate halt to all violence’.
The administration of US President George Bush, in its final weeks in office, appeared to put the onus on Hamas to prevent a further escalation.
‘Hamas' continued rocket attacks into Israel must cease if the violence is to stop,’ White House spokesman Gordon Johndroe said in a statement that urged Israel to avoid civilian casualties but stopped short of calling for an end to the Israeli air strikes.
EU Foreign Policy Chief Javier Solana, in contrast, called for an immediate ceasefire and urged ‘everybody to exert maximum restraint’, his spokesman said.
The Minister for Foreign Affairs, Micheál Martin, has strongly condemned Israel's actions, saying they were likely to escalate the cycle of violence in the Middle East.
But the Israeli ambassador to Ireland, Dr Zion Evrony, said his government had to attack in order to protect its own citizens
Hamas has estimated that at least 100 members of its security forces were killed, including police chief Tawfiq Jabber and the head of Hamas's security and protection unit, along with at least 15 women and some children.
