Israel’s Prime Minister Benjamin Netanyahu said Monday that the Israeli military would attack Hamas with a force “like never before,” as the militant group threatened to kill civilian hostages if airstrikes target Gaza without warning.

Following Hamas’ devastating surprise attack in Israeli territory over the weekend, Israeli defense minister Yoav Gallant on Monday also ordered a “complete siege” of Gaza, and said he would halt the supply of electricity, food, water and fuel to the Palestinian enclave.

“I have given an order – Gaza will be under complete siege,” the minister said. “We are fighting barbarians and will respond accordingly.” Israel has been pounding what it describes as “strategic” Hamas locations in Gaza with airstrikes since the Hamas attack.

Hours later, a spokesperson for Hamas’ armed wing said it would begin killing civilian hostages and broadcasting the act if Israel targets people in Gaza without warning. “We declare that we will respond to any targeting of our people who are safe in their homes without warning, with the execution of our civilian hostages, and we will broadcast it with audio and video,” Abu Obaida said in a statement on the Al-Qassam Brigades’ Telegram channel.

Hamas launched its unprecedented surprise assault early Saturday, firing thousands of rockets and sending armed fighters into Israel. The attack has killed at least 900 people in Israel and injured thousands, Israel’s Army Radio reported Monday.

In response, Israel on Sunday formally declared war on Hamas.

Israeli jets continued to bombard Gaza with deadly airstrikes Monday as the violence continued into a third day. The strikes have killed at least 687 people, including dozens of children and women, and left thousands injured, according to the Palestinian Ministry of Health.

Attacks on suspected Hamas strongholds in Gaza is “just the beginning,” Netanyahu warned during a televised speech on Monday. “I said that every place from which Hamas operates will turn into ruins. It is already happening today, it will happen even more in the future,” he said.

“We are fighting for our home and for our existence,” he also said.

Israel’s military has already retaken control of Israeli communities that were stormed by the militant group’s gunmen over the weekend, and there is no ongoing fighting between Israeli forces and Hamas militants inside Israel, it said.

At least 11 US citizens have been confirmed killed in Israel, US President Joe Biden said Monday.

It is also “likely” that Americans are among those being held hostage, Biden said, adding that his administration is working with Israeli officials on “every aspect of the hostage crisis.”

Pleas for hostages’ safe return

Hamas militants claimed late Sunday to be holding more than 100 hostages in Gaza, including high-ranking Israeli army officers, according to Mousa Abu Marzouk, chief deputy of Hamas’ political bureau.

Videos on social media showed militants capturing multiple civilians, including children, as Israeli families across the nation made anxious pleas for the safe return of their loved ones.

In addition to Israeli captives, other nationalities are also believed to have been taken hostage, including American, Mexican, Brazilian and Thai nationals – further complicating Israel’s response to the Hamas attack. At least nine citizens from Peru, Paraguay, Brazil and Mexico are missing, according to their respective authorities.

Abu Obaida, the Al-Qassam Brigades spokesperson, said Hamas will not negotiate on the issue of hostages while under Israeli fire.

“It has become clear that the enemy’s hostages are at risk to the same extent as our people in light of the aggression against the Gaza Strip,” Abu Obaida said in a statement. “We affirm that we will not deliberate or negotiate on the issue of hostages under fire, in light of aggression, or in light of battle.”

Earlier, IDF spokesperson Lt. Col. Richard Hecht was asked whether it had stopped warning civilians before it bombs a building, known as the “knock on the roof.”

Hecht responded that Hamas did not provide any warning.

“When they came in and threw grenades at our ambulances they did not knock on the roof. This is war. The scale is different,” Hecht said.

‘Massacre’ in Gaza

Airstrikes have been Israel’s primary retaliation measure within Gaza itself, with jets repeatedly pounding the heavily populated 140 square mile coastal strip, turning multiple buildings to rubble, displacing tens of thousands of people and sending waves of injured Palestinians to overwhelmed hospitals.

An IDF spokesman said it had been hitting Hamas, destroying around 800 targets and killing “hundreds” of fighters, wounding thousands and capturing scores of others.

Most of those arriving at hospitals in Gaza have sustained second- and third-degree burns and amputations, a spokesperson for the Palestinian health ministry in Gaza told Palestinian news outlet Shihab Agency on Monday. Many have also sustained shrapnel injuries, Ashraf al-Qidra said.

Those seeking hospital care are mainly women and children, al-Qidra said, adding that this is a “result of Israelis directly targeting residential houses and buildings.”

Access to medical care has been complicated by Israel cutting power to the territory, threatening the “lives of hundreds” of those injured, the Palestinian Ministry of Health in Gaza said.

The ministry said later that all services at the only functioning hospital in Gaza’s Beit Hanoun neighborhood were suspended due to continuous Israeli airstrikes, blocking medical teams’ ability to enter or exit the building. Nine ambulances have been targeted since Saturday, the ministry added.

Israeli airstrikes targeted the Shati and Jabalia refugee camps in Gaza on Monday, the Palestinian health ministry said, describing the assault as a “massacre against the entire neighborhood.”

The ministry said bodies were still being recovered after the strikes killed a “large number” of people. No death toll has been provided.

Meanwhile, Palestinian Minister of Health Mai al-Kaila urged the international community to stop “the aggression” against medical facilities and teams in Gaza.

At least 13 family members, including four toddlers, were killed in an Israeli airstrike in Khan Younis in southern Gaza on Sunday, according to journalist Hassan Eslayeh and a family relative.

Sounds of battle

While it remains unclear what the full scale of the Israeli response will be, Prime Minister Benjamin Netanyahu on Sunday predicted a “long and difficult war” and vowed “mighty vengeance” on Hamas.

Hecht said Sunday that the IDF’s priority for the coming hours and days was to “control the entire enclave and kill all the terrorists in our territory.”

Israel’s declaration of war set the stage for a major military operation in Gaza, and tanks and personnel carriers could be seen on the move near the Israel-Gaza border on Sunday.

Thousands of Israeli reservists have been called up and the IDF announced that several communities close to the Gaza security fence are being evacuated.

An Israeli military official and a United States defense official said Israel is requesting precision guided bombs and additional interceptors for its Iron Dome missile system from the US, including Joint Direct Attack Munitions, or JDAMs – a kit that turns an unguided “dumb” bomb into a precision “smart” weapon.

US Defense Secretary Lloyd Austin said the US would provide security assistance to Israel imminently. The US said it was also sending a Navy carrier strike group to the eastern Mediterranean Sea, including guided missile destroyers and guided missile cruisers.

Horror on the ground

Many Israelis have spent much of the past two days in bomb shelters and saferooms.

Throughout the bloody weekend, Hamas rockets made direct hits on multiple locations inside the country, including Tel Aviv, while armed terror groups entered Israel and infiltrated military bases, towns and farms, shooting at civilians and taking hostages.

The assault has left Israel reeling and impacted families far beyond its borders. Twelve Thai citizens, 10 Nepalis, two Ukrainians, two French nationals and one British citizen are among those killed in Israel.

Photos released by the Israeli foreign ministry showed dozens of bodies in the aftermath of Hamas gunmen’s attack on a music festival near the Israel-Gaza border, which emergency responders said left at least 260 dead.

“One couldn’t describe it with words. It’s impossible… It was a very difficult moment,” Yakov Argamani said, describing the moment he saw the video of his 25-year-old daughter Noa for the first time.

The IDF said early Monday that Be’eri was “very badly hit.”

Regional concerns of escalation

Questions remain over how the Israeli military and intelligence apparatus appeared to be caught off guard in one of the country’s worst security failures.

Fighting between the two sides has surged in the past two years. The violence has been driven by frequent Israeli military raids in Palestinian towns and cities, which Israel has said are a necessary response to a rising number of attacks by Palestinian militants on Israelis.

UN peacekeepers urged restraint after the conflict flared out into the wider region on Monday, when the IDF said it killed armed individuals who “infiltrated” Israel from Lebanon after the Lebanese group Hezbollah claimed responsibility for targeting three Israeli sites in Shebaa Farms, which Lebanon considers Israeli-occupied.

On Sunday, the UN Security Council held an emergency meeting but no action was taken afterward.

European Union foreign ministers are expected to meet Tuesday to address the situation in Israel.