By Hadas Gold, Eyad Kourdi, Jonny Hallam, Ibrahim Dahman and Helen Regan, CNN

Jerusalem and Gaza (CNN) — Israel’s defense minister ordered a “complete siege” of Gaza on Monday, as the military said it had retaken control of Israeli communities near the coastal enclave that was stormed by Hamas gunmen in an unprecedented attack over the weekend.

Yoav Gallant said Israel would halt the supply of electricity, food, water and fuel to Gaza. “I have given an order – Gaza will be under complete siege,” Gallant said. “We are fighting barbarians and will respond accordingly.”

Israel has retaken control of all communities around Gaza and there is no ongoing fighting between Israeli forces and Hamas militants inside Israel, the Israeli military said on Monday, following continued assaults by both sides.

The announcement by Israel Defense Forces (IDF) spokesperson Rear Adm. Daniel Hagari came more than 48 hours after Hamas launched a surprise assault with thousands of rockets and sent armed fighters into Israel.

Israel on Sunday formally declared war on the Islamist militant group Hamas after its fighters launched an unprecedented attack that has so far killed more than 700 people in Israel. Israeli jets continued to bombard the Palestinian enclave of Gaza on Monday as the war entered its third day.

Israeli strikes in Gaza have killed at least 493, including dozens of children, and left 2,651 injured, according to the Palestinian Ministry of Health.

An incursion by Gaza militants of this scale has not been seen in Israel since the nation’s founding in 1948.

Earlier Monday, as Israeli forces battled to expel the last assailants, Hamas launched a fresh barrage of rocket attacks. Sirens warning of incoming rockets in Jerusalem and Tel Aviv sounded at noon local time (5 a.m. ET).

Hamas said it fired 120 rockets toward the coastal cities of Ashdod and Ashkelon, in response to Israeli airstrikes. A CNN team on the ground saw dust billowing over the sky, as rockets launched from Gaza were intercepted by the Israeli air defense system in Ashdod.

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, there are also other nationalities believed to have been taken hostage, including American, Mexican, Brazilian and Thai nationals – further complicating Israel’s response to the Hamas attack.

For now, airstrikes have been the primary retaliation measure within Gaza itself, with Israeli jets repeatedly pounding the heavily populated 140 square mile coastal strip, turning multiple buildings to rubble.

The IDF says it has been hitting Hamas, destroying around 800 targets and killing “hundreds” of fighters, wounding thousands and capturing scores of others, spokesperson Hagari said Sunday.

Tens of thousands of people have been displaced in Gaza, the United Nations Relief and Works Agency said, and 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.

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.

Sounds of battle

IDF spokesperson Lt. Col. Richard Hecht said on Sunday that the priority for the coming hours and days was to “control the entire enclave and kill all the terrorists in our territory.” Israeli fighters continued to clash with Hamas militants on Israeli soil earlier Monday morning in up to eight locations.

CNN reporters on the ground heard repeated sounds of battle throughout the night as well as Israel’s Iron Dome missile system firing to intercept Hamas rockets.

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 Iron Dome interceptors 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 will 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.

Israel’s Ambassador to the US, Michael Herzog, told CNN Hamas fired more than 4,000 rockets at Israel and sent hundreds of militants into Israeli territory.

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.

Among those killed in Israel are 12 Thai citizens, 10 Nepalis, four US nationals, two Ukrainians, one French and one British citizen.

Images and videos show the horror unfolding on the ground.

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

The father of an Israeli woman who was reportedly taken hostage at the festival told CNN that he “didn’t want to believe it” when he saw his daughter being hoisted onto the back of a motorcycle by Hamas militants in a video circulating on social media.

“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.

Videos obtained and geolocated by CNN show at least four civilians in the kibbutz of Be’eri were killed while in the custody of Hamas, just feet from where armed militants had been escorting them.

The IDF said early Monday that Be’eri was “very badly hit,” and although most Hamas militants in the kibbutz had been killed, Israeli troops were still fighting there.

Israeli warplanes continued striking Gaza overnight Sunday into Monday and “severely degraded the capabilities” of Hamas, the IDF said. Among the targets was a structure that housed Hamas operatives and several Hamas command centers, including one belonging to a senior operative of the Hamas naval forces.

The barrage of Israeli strikes in Gaza has also inflicted casualties on civiliansin what is one of the world’s most densely populated places.

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

Regional concerns of escalation

The attacks come after months of surging violence between Palestinians and Israelis, with the long-running conflict now heading into uncharted and dangerous new territory. 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.

Hamas’ highly coordinated assault, which began Saturday morning, was unprecedented in its scale and scope and came on the 50th anniversary of the 1973 War in which Arab states blitzed Israel on Yom Kippur, the holiest day of the Jewish calendar.

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.

Concerns the conflict could spill out into the region were raised Sunday when Lebanese group Hezbollah claimed responsibility for targeting three Israeli sites in an area known as Shebaa Farms, using missiles and artillery. The area is considered by Lebanon as Israeli-occupied.

The IDF said its artillery struck the area in Lebanon where the firing originated and said it “will continue to operate in all regions and at any time necessary to ensure the safety of the Israeli civilians.”

On Sunday, the UN Security Council held an emergency meeting but no action was taken afterward. The Deputy US Ambassador to the UN said “not all” the member nations had condemned Hamas’ attacks, but did not specify which. All 15 members need to vote unanimously for the UNSC to release a statement.