The UN Security Council is meeting tonight to consider an Egyptian request for an emergency meeting on Israel's military action in Gaza.
The closed-door meeting comes in response to a letter from Egyptian Ambassador Muutaz Khalil.
Secretary-General Ban Ki-moon released statements saying he had spoken to Egyptian President Mohamed Morsi about "the worrisome escalation of violence in southern Israel and Gaza and the need to prevent any further deterioration."
He also said he had talked to Israeli Prime Minister Benjamin Netanyau to express concern about "an alarming escalation of indiscriminate rocket fire from Gaza into Israel and the targeted killing by Israel of a Hamas military operative in Gaza."
Israel killed Hamas commander Ahmad Jabari in one of some 20 airstrikes on the Gaza Strip Wednesday, the heaviest barrage on the Palestinian territory in four years in retaliation for renewed rocket fire on southern Israel.
Gaza's health minister said 10 people were killed — two of them young children — while the Israeli military said its attack was just the beginning of a major offensive and warned it could escalate with a ground attack.
Renewed rocket fire
Palestinian militants responded to the attack with renewed rocket fire. The military said its "Iron Dome" defence system intercepted 13 rockets from Gaza. Israeli media said the rockets were shot down over the city of Beersheba.
Netanyahu said Israel could not tolerate continued rocket attacks against its citizens and said the military was prepared to broaden its operation against Hamas targets in Gaza.
"We will continue to do everything to protect our citizens," he said.
The military said earlier the operation could escalate with a ground attack.
Canada backs Israel's actions
The Canadian government said following Wednesday's rocket attacks that it will continue to support Israel.
"We fundamentally believe that Israel has the right to defend itself and its citizens from terrorist threats," Rick Roth, a spokesman for Foreign Affairs Minister John Baird, said in a statement.
"Far too often, the Jewish people find themselves on the front lines in the struggle against terrorism," Roth said.
The White House said President Barack Obama spoke with Netanyahu Wednesday night and reiterated U.S. support for Israel's right to self-defence from rocket attacks being launched against its civilians and urged Israel to "make every effort to avoid civilian casualties" in its response.
Obama and Netanyahu agreed that Hamas needs to stop its attacks on Israel to allow tensions to ease, the White House said. The two leaders agreed to stay in close touch in the coming days.
Obama spoke separately to Morsi, given Egypt's central role in preserving regional security, the White House said. The two men agreed on the need to de-escalate the conflict as quickly as possible.
Miriam Ziv, Israeli ambassador to Canada, talked about the military action to host Evan Solomon on CBC News Network's Power & Politics.
"From last Saturday on until today, 120 rockets were shot towards Israeli citizens in the southern part of Israel," she said.
"They have been targeting endlessly, daily … Israeli citizens, Israeli villages and cities and towns through all these months. And the last few days were really unbearable for the citizens in Israel."
Israel had also gathered intelligence that Hamas has amassed a "huge number" of missiles, some with a range of 75 kilometres, she said.
The offensive followed a weekend exchange of rocket fire from Gaza on southern Israel and Israeli airstrikes. Seven Palestinians were killed and several Israelis wounded.
The military said its aircraft targeted dozens of facilities that served as storage or launching sites for rockets.
Responding to questions on Power & Politics about civilians being killed by Israeli strikes, Ziv said civilian deaths are regrettable.
"But we know the strategy and the tactics of the Hamas … They actually keep their arsenal inside the civilian population and unfortunately innocents are being hurt, but this is not the aim," she said.
Jabari topped Israel's most-wanted list
The killing of Jabari marked a dramatic resumption of Israel's policy of assassinating Palestinian militant leaders. He was the most senior Hamas official to be killed since the last war in Gaza ended in early 2009. He has long topped Israel's most-wanted list, blamed for a string of deadly attacks, including the kidnapping of Israeli soldier Gilad Schalit in 2006.
Israeli Defence Minister Ehud Barak said the operation in Gaza was far from over and that airstrikes had already inflicted heavy damage, taking out most of Hamas' long-range missile capabilities. Plumes of smoke wafted into Gaza City's skies following at least five airstrikes there.
Gaza Health Minister Dr. Mufeed Mkhallalati said a total of 10 people were killed, including Jabari, and 45 wounded, 10 of them in critical condition. Among the dead were three civilians, including an 11-month-old and a six-year-old.
Palestinians call for retaliation
Palestinians called for harsh retaliation. Hamas, which rules Gaza, announced a state of emergency in the territory and evacuated all security buildings.
"The battle between us and the occupation is open and it will end only with the liberation of Palestine and Jerusalem," Hamas official Khalil al-Haya said.
Witnesses said Jabari was travelling in a car in Gaza City when the vehicle exploded. Crowds of people and security personnel rushed to the scene of the strike, trying to put out the fire that had engulfed the car and left it a charred shell. The Israeli military released a grainy, black-and-white video of the airstrike.
Fearing a long war in Gaza, Palestinians rushed to buy fuel, bread and other food supplies.
"We are working under fire to protect our people and to back the resistance," said Islam Shahwan, a Hamas interior ministry spokesman. "We have a full emergency plan that we are adopting now."
Israeli military spokeswoman Lt. Col. Avital Leibovitch said Jabari had "a lot of blood of his hands" and that the military chief "authorized different targets" as well.
Strikes called extrajudicial killings by critics
Advocates say targeted killings are an effective deterrent without the complications associated with a ground operation, chiefly civilian and Israeli troop casualties. Proponents argue they also prevent future attacks by removing their masterminds.
Critics say the killings invite retaliation by militants and encourage them to try to assassinate Israeli leaders. They complain that the strikes amount to extrajudicial killings.
"Assassinating leaders is never the solution. In place of the leaders killed, others will grow, and we will only get another cycle of fire and blood," said Israeli lawmaker Dov Hanin.
During a wave of suicide bombings against Israel a decade ago, the country employed the tactic of using aircraft to eliminate the upper echelon of Hamas leadership.
That set off a wave of criticism from rights groups and foreign governments, particularly over a bomb that killed a Hamas leader and 14 other people, most of them children.
Israeli opposition leader Shaul Mofaz, a former chief of staff who has supported targeted killings, welcomed the strike.
"We need to continue this policy, to find them in every place," he told Israel's Army Radio. "Israel needs to determine the agenda, not Jabari."
Escalation in violence expected
Mofaz warned that Israelis should expect an escalation of violence in the coming days.
Anticipating more Hamas retaliation, the military issued instructions to citizens in southern Israel to stay close to home and school was cancelled for Thursday.
"Jabari was a most extreme man and was responsible for all the attacks and assassinations from Gaza against Israel," President Shimon Peres said. "We shall handle it with great care. Our intention is not to raise the flames, but already for days, day and night, they are shooting rockets at Israel."
Jabari, nicknamed Abu Mohammed, was born in 1960 in the eastern Gaza neighbourhood of Shejaiya.
He survived four previous attempts by Israel to kill him. In one attempt in 2004 his eldest son, his brother and three other relatives were killed.
He was said to have led the bloody 2007 takeover of Gaza from Fatah forces, developing Hamas's military arsenal and its networks in Iran, Sudan and Lebanon. Hamas has ruled Gaza with an iron grip since then.
The assassination threatened to further damage Israel's relations with Egypt, which is governed by Hamas's ideological counterpart, the Muslim Brotherhood.
On its official Facebook page, the Freedom and Justice Party, the Muslim Brotherhood's political arm, called the assassination a "crime that requires a quick Arab and international response to stem these massacres against the besieged Palestinian people in the Gaza Strip."
It accused Israel of trying to "drag the region toward instability."
Israel and Egypt signed a peace accord in 1979. Relations, never warm, have deteriorated since longtime Egyptian president Hosni Mubarak was ousted in a popular uprising last year.