Israeli Prime Minister Banjamin Netanyahu has said that Israel's fierce bombardment of Gaza was "just the beginning" of his country's response to a Hamas onslaught that left 1,300 dead.
With tens of thousands of Israeli troops massed on the Gaza frontier, Mr Netanyahu said in televised address: "Our enemies have only just started paying the price.
"I cannot reveal what will happen, but I am telling you this is just the beginning."
He spoke only hours after the Israeli military revealed that troops had carried out localised raids inside the besieged Palestinian territory over the past 24 hours as a prelude to an anticipated ground invasion.
Israel has also told Palestinians to leave Gaza City within 24 hours, sparking UN warnings of a humanitarian disaster.
Mr Netanyahu reaffirmed his pledge that Hamas would be "destroyed".
"We will never forgive, we will never let the world forget these horrors inflicted on the Jewish people. We will fight our enemies using a power without limit."
The deadly attack which Hamas gunmen launched last Saturday also saw around 150 Israelis and foreigners abducted to Gaza as hostages.
Gaza officials say that retaliatory Israeli shelling and air raids have killed about 1,800 people in the Hamas-ruled territory.
Evacuation of hospital patients 'impossible' - WHO
Earlier, the World Health Organization said that local health authorities in Gaza had informed it that it was impossible to evacuate vulnerable hospital patients from northern Gaza after Israel's military called for civilians to relocate south within 24 hours.
"There are severely ill people whose injuries mean their only chances of survival is being on life support, such as mechanical ventilators," said WHO spokesperson Tarik Jasarevic.
"So moving those people is a death sentence. Asking health workers to do so is beyond cruel."
WHO, which already warned that hospitals in Gaza are at breaking point, said hospitals in the south were overflowing, while the two major hospitals in the north had already exceeded their combined 760-bed capacity.
"Hospitals have only a few hours of electricity each day as they are forced to ration depleting fuel reserves and rely on generators to sustain the most critical functions," Mr Jasarevic said. "And even those functions will have to cease in a few days when fuel stocks run out."
He added that this would have a devastating impact on injured people requiring surgery, patients in intensive care units and newborns reliant on incubators.
Gaza is also experiencing a shortage of hospital blood banks and medicine, the WHO said.
"Time is running out to prevent a humanitarian catastrophe, if fuel, water, food and life-saving health and humanitarian supplies cannot be urgently delivered to the Gaza Strip amidst the complete blockage," Mr Jasarevic said.
Hamas rejects Israel call to evacuate Gaza City
Hamas has rejected the Israeli order for 1.1 million residents to evacuate northern Gaza within 24 hours ahead of an expected ground incursion into the overcrowded Palestinian territory.
"Our Palestinian people reject the threat of the occupation (Israeli) leaders and its call for them to leave their homes and flee from them to the south or Egypt," Hamas said in a statement.
"We are steadfast on our land and in our homes and our cities. There will be no displacement," it said.
Egypt's Rafah crossing is the only route out of Gaza not controlled by Israel and has been bombed on several occasions this week.
President Abdel Fattah al-Sisi said Egypt remains committed to ensuring the delivery of aid to Gaza, but urged Palestinians to "remain on their land" in a speech yesterday.
Israel's military called for all civilians of Gaza City, to relocate south, as it amassed tanks near Gaza after a devastating attack by the militant group Hamas.
Tánaiste Micheál Martin has said that Israel has a right to self-defence but it must act within international law.
Speaking in University College Cork this morning, Mr Martin said that it is not feasible that a million people in Gaza can leave in 24 hours.
He said that call by Israel should be rescinded, adding "everybody knows it's not something that can be achieved at all".
The Tánaiste said two wrongs do not make a right and there was an obligation to protect civilians in Gaza, "people who have nothing to do with Hamas".
Mr Martin said Ireland was working with the United Nations "in terms of Mediterranean corridors", saying that it was essential "that significant amounts of humanitarian aid needs to get into Gaza."
"Now is a time for war," Defence Minister Yoav Gallant said yesterday as Israeli warplanes continued pounding Gaza in retaliation for the deadliest attack by Palestinian militants in its history.
The Israeli military said it would operate "significantly" in Gaza City in the coming days and civilians would only be able to return when another announcement was made.
The United Nations said it considered it impossible for such a movement to take place "without devastating humanitarian consequences".
"The United Nations strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation," UN spokesman Stephane Dujarric said in a statement.
Israel's ambassador to the UN Gilad Erdan described the UN's response to Israel's early warning to the residents of Gaza as "shameful".
Israel has vowed to annihilate the Hamas militant group which led the attacks on Saturday, but a ground invasion of Gaza poses serious risk with Hamas holding scores of hostages kidnapped in the assault.
Israel has put Gaza, home to 2.3 million people, under siege.
The International Committee of the Red Cross said fuel powering emergency generators at hospitals in Gaza could run out within hours and the United Nations World Food Programme warned food and fresh water were running dangerously low.
"The human misery caused by this escalation is abhorrent, and I implore the sides to reduce the suffering of civilians," ICRC regional director Fabrizio Carboni said.
Seeking to build support for its response, Israel's government showed US Secretary of State Antony Blinken and NATO defence ministers graphic images of children and civilians they said Hamas had killed in a weekend rampage in Israel.
Mr Blinken said they showed a baby "riddled with bullets," soldiers beheaded and young people burned in their cars. "It's simply depravity in the worst imaginable way," he said. "It's really beyond anything that we can comprehend."
Like others across the globe, Mr Blinken urged Israel to show restraint, but he also reiterated the US's support, saying: "We will always be there by your side."
Mr Blinken was due to meet Jordan's King Abdullah and Mahmoud Abbas, head of the Palestinian Authority in the Israeli-occupied West Bank, as part of a Middle East tour aimed at stopping spillover from the war.
He also planned to visit key US allies Qatar, Saudi Arabia, Egypt and the United Arab Emirates - some with influence on Hamas, an Islamist group backed by Iran.
Israel's military chief, Lieutenant General Herzi Halevi, said lessons would be drawn from the security failures around Gaza that enabled the attack.
"We will learn, investigate, but now is the time for war," he said.
