Israel trying to avoid Gaza civilian casualties, says PM Netanyahu
Israeli PM Benjamin Netanyahu says Israel will do "everything in its power" to avoid civilian casualties in the conflict with Hamas.
He accused Hamas of the "double war crime" of firing at Israeli civilians and hiding behind Gaza civilians.
Three Israelis have been killed by rocket fire from Gaza, while 15 Palestinians have been killed in two days of Israeli attacks on Gaza.
Egypt's PM is to travel to Gaza in a show of support, sources say.
Hisham Qandil will arrive in the territory on Friday, Egyptian diplomatic sources told the BBC.
Israel's attacks began with the assassination of Hamas military chief Ahmed Said Khalil al-Jabari, whose car was destroyed by a missile in Gaza City.
His death on Wednesday followed a wave of rocket fire from Hamas and other Palestinian militant groups in Gaza targeting southern Israel.
Gaza - 15 dead, more than 130 injured, including:
- Hamas military chief Ahmed Said Khalil al-Jabari and bodyguard, killed in Israeli missile strike on their car in Gaza City
- BBC Arabic picture editor's 11-month-old son and sister-in-law, killed in air strikes on Gaza City; son and brother and uncle critically injured
- Child who died from injuries after raids on Khan Younis
- Two people killed in Beit Lahiya
- Woman pregnant with twins killed - location unknown
- Casualties reported in Rafah
Israel - three killed, six injured:
- Two women and man who died when rocket hit residential building in Kiryat Malachi; another three injured there
- Three Israeli soldiers reportedly wounded in rocket attacks on Eshkol
Sources: Palestinian police and medics/Israeli officials
Since the killing, southern Israel has been targeted by about 250 rockets, Israel Army Radio reports.
Israel's Iron Dome anti-missile defence system is said to have intercepted 92 of them.
One rocket reached as far as the city of Rishon LeTzion, some 15km (nine miles) south-east of Tel Aviv, but there were no injuries or damage there, Army Radio said.
Mr Netanyahu said Hamas was deliberately placing its rockets and missiles in civilian areas prior to launching them at Israel.
"There is no moral symmetry between Israel and the terrorists in Gaza," he said during a televised news conference in Tel Aviv.
"Hamas deliberately targets our children and they deliberately place their rockets next to their children. Now despite this reality - and it's a difficult reality - Israel will continue to do everything its power to avoid civilian casualties."
He added that Israel would take "whatever action is necessary" to defend its citizens.
The three Israelis - two women and a man - were killed on the top floor of a four-storey building in the town of Kiryat Malachi, about 25km (15 miles) north of Gaza, that was hit on Thursday morning. They were the first Israeli fatalities in this latest round of violence.
Three other people - a four-year-old boy and two babies - were injured, Israel's foreign ministry said.
Many of the 15 Palestinians killed were members of militant groups, but civilians - including four children - were also among the dead. They included 11-month-old Omar, the son of Jihad Misharawi, a BBC Arabic picture editor.
BBC reporters on the crisis
The danger of the kind of operation Israel has started is that rising casualties on both sides cause a violent escalation that neither side can control. If that happens it could cause a much bigger crisis across the Middle East.
The overwhelming mood in Gaza is sombre but vengeful. Sombre because few here want another war with Israel - another war in which civilians would undoubtedly suffer as much as the fighters.
There is a gaping hole on the top floor of an old four-storey apartment building where two Israeli families were living. A resident Yerumichael Simon told me: "I live across the street and I used to live inside that building."
Egyptian President Mohammed Mursi has condemned the air strikes after recalling his ambassador from Israel.
"The Israelis must understand that we do not accept this aggression," he said in televised remarks.
Egyptian Foreign Minister Mohammed Kamel Amr spoke to US Secretary of State Hillary Clinton and called for "immediate US intervention to stop the Israeli aggression".
For its part, the US has publicly backed Israel, with state department spokesman Mark Toner saying: "We support Israel's right to defend itself, and we encourage Israel to continue to take every effort to avoid civilian casualties."
British Foreign Secretary William Hague said that "Hamas bears principal responsibility for the current crisis", also urging all the sides involved to "avoid any action which risks civilian casualties or escalates the crisis".
States of emergency have been declared on both sides of the Israel-Gaza border, with civilians taking shelter from rocket and missile fire.
Hamas evacuated all its security buildings in Gaza, while Israeli aircraft dropped leaflets warning Gazans to stay away from Hamas, other militants and their facilities.
Nonetheless, thousands of people - including top Hamas officials - attended Ahmed Jabari's funeral in Gaza City, where gunmen fired automatic weapons into the air.
"This crime will not weaken us. It will make us stronger and more determined to continue the path of jihad and resistance," Hamas lawmaker Mushir al-Masri said in a eulogy. "The enemy opened the battle and shall bear the consequences."
In southern Israel, where more than a million Israelis live within rocket range, schools and shopping centres were closed and people living in areas along the frontier were ordered to stay at home instead of going to work.
Israel's aerial and naval bombardment of the Hamas-run Gaza Strip is its most intense assault on the Palestinian territory since it launched a full-scale invasion four years ago, also in response to rocket fire.
Palestinian President Mahmoud Abbas - who lost control of Gaza to Hamas in 2007 and now governs only the West Bank - has cut short a trip to Europe in response to the crisis and returned to the Middle East.
Arab League foreign ministers say they will meet in emergency session on Friday to discuss the escalating violence.