On Saturday, Hamas launched an incursion out of Gaza on Israel, escalating a long-running conflict.
Israeli Prime Minister Benjamin Netanyahu formally declared war on the Islamist militant group on Sunday. Leading the Israeli military to launch strikes at Gaza City.
The death toll from the two-day conflict has surpassed 1,500 on both sides and thousands more have been wounded.
More than 10 Britons are feared dead or missing in Israel, ITV News understands.
Hamas have released a statement detailing their plan to execute hostages if Palestinian civilians are 'targeted without prior warning'.
Israel has ordered a "complete siege" on Gaza, with authorities cutting off electricity and blocking the entry of food and fuel, Defence Minister Yoav Gallant has said.
The Israeli government has been intensifying its bombardment of the Gaza Strip after declaring it is "at war" against Hamas - after the militant group attacked Israel on Saturday.
The death toll from the two-day conflict has surpassed 1,600 and thousands others have been wounded on both sides.
At least 900 people have reportedly been killed in Israel and over 680 have been killed in Gaza.
The Palestinian militant group claims to be holding more than 130 captives from the Israeli side, and announced on Monday evening via platform Telegram that, if their civilians are targeted, Hamas will execute the hostages on camera.
"They have shelled and destroyed many residential homes. So, we, from this hour onwards, have decided and announce that any targeting of civilians in their homes without prior warning will be met, regrettably, with an execution of one of the civilian hostages that are held by us. We are being forced to do so," the message said.
More than 10 Britons are feared dead or missing in Israel, ITV News understands. The US State Department has confirmed that nine citizens have been killed in the attacks.
Three British citizens have been confirmed dead so far, Nathanel Young who was serving with the Israeli Defence Services, Bernard Cowan, a Scottish man in his 50s, and Dan Darlington, a photographer.
Israeli Prime Minister Benjamin Netanyahu declared his country is "at war" and warned the military will use all of its strength to punish those responsible for Saturday's assault.
Whereabouts did Hamas strike and how did the attack unfold?
Speaking in a televised address vowed vengeance saying: "We have only started striking Hamas. What we will do to our enemies in the coming days will reverberate with them for generations."
President Joe Biden confirmed US an aircraft carrier and numerous support vessels are en route to the Eastern Mediterranean ready to support Israel.
The carrier - the USS Gerald R. Ford - is the largest aircraft carrier in the world and the largest warship ever constructed.
After an initial incursion into Israeli territory that saw several towns swarmed with militants CNN has reported the Israel Defence Force is now only fighting around the Gaza Strip.
Prime Minister Rishi Sunak chaired a Cobra meeting on Monday afternoon to discuss the crisis in Israel after days of escalating violence.
He said the "barbaric acts" committed by Hamas were "evil."
It is believed over 100 Israelis may be held hostage by Hamas, grandma of six Adva is among them
He said: "Teenagers at a festival of peace gunned down in cold blood. Innocent men, women and children abducted, raped, slaughtered."
The PM added: "We stand with Israel, the United Kingdom stands with Israel against this terrorism today, tomorrow and always."
The leaders of the UK, US, Germany, France and Italy - including Sunak, Biden, Olaf Scholz, and Emmanuel Macron - also issued a joint statement promising "steadfast and united support" to Israel.
They said they recognised the “legitimate aspirations of the Palestinian people”, but “Hamas does not represent those aspirations, and it offers nothing for the Palestinian people other than more terror and bloodshed”.
Around 50,000 to 60,000 British nationals are believed to be in either Israel or Gaza, the government estimates, amid concerns for their safety following the latest upsurge in violence in the region.
Families of loved ones being held hostage have detailed the horror to ITV News, as Sejal Karia and John Irvine report
Downing Street said the UK supports Israel taking proportionate action in response to the attack by Hamas, amid ongoing Israeli airstrikes on Gaza but it is not planning to deploy military assets to the Middle East.
The Foreign Office has advised against all but essential travel to Israel and Occupied Palestinian Territories due to the conflict.
Scotland's First Minister Humza Yousaf has said his parents-in-law are "trapped" in Gaza after visiting family, leaving him worried if they will "make it through the night".
They went to visit Mr Yousaf’s father-in-law’s 92-year-old "elderly and frail" mother when the Hamas attack took place and have been told by Israeli authorities to leave because “Gaza will effectively be obliterated”, the first minister said.
On Monday, a crowd gathered outside Downing Street to hold a vigil for the outbreak of violence in Israel.
At the same time in Kensington, a large crowd gathered chanting "free Palestine."
Large groups of police officers watched on as placards calling for Israel to "end the occupation" were waved.
Miranda Janowski, a protester at the vigil, told ITV News her heart is "torn into a thousand pieces" after hearing the news of the fatalities at the Supernova festival near the Gaza-Israel border.
"I go to these parties all the time, I actually went to one two weeks ago, a lot of my friends were there, some escaped.
"Hearing their stories about what they've seen and knowing that 260 beautiful souls, because they're beautiful people that go to these parties, were murdered in cold-blood, my heart is torn into a thousand pieces."
Hamas attacked Israel by land, air and sea on Saturday, prompting Jerusalem to launch its own barrage of attacks on Sunday battling fighters in the streets and striking Gaza from the skies.
Footage captured by the Palestinian Civil Defence services shows an emergency rescue operation in Gaza following a missile strike
Videos on social media showed militants capturing multiple civilians, including children, as Israeli families across the nation made anguished pleas for the safe return of their loved ones.
Several people from other countries including the US, Mexico and Brazil have also been confirmed as being taken hostage by Hamas.
The best friend of a 25-year-old student, named Noa Argamani, who was taken with her boyfriend - with Hamas believed to be behind the kidnap - told ITV News she fears the worst and doesn't "even want to imagine what they're doing to her".
Harrowing footage shared by Noa's family showed her and her boyfriend pleading for their lives while she was taken away on the back of a motorbike. They haven't heard from her since.
Noa's friend, Shlomit Marciano, told ITV News: "I can't get it out of my head. She's screaming 'don't kill me'.
"Terrible, terrible. Last time I saw her she was happy - now that video. It's crazy."
She said Noa had hidden for hours from Hamas gunmen who attacked a party attended by hundreds of young people, with many killed.
Shlomit said "most likely" her friend is now in the hands of Hamas.
When asked what she fears, she replied: "I think you know what I fear - that she's been hurt. I don't even want to imagine what they're doing to her."
'I don't even want to imagine what they're doing to her', Noa's friend, Shlomit Marciano, told ITV News
Hamas launched Saturday's attack by using explosives to break through the border fence enclosing Gaza, before crossing with motorcycles, pickup trucks, paragliders and speed boats on the coast.
The gunmen infiltrated up to 22 locations, as thousands of missiles were aimed at major Israeli cities.
Hamas' military wing leader, Mohammed Deif, said the assault was part of "Operation Al-Aqsa Storm", and called on all Palestinians to join the fight.
The EU Commission has said it is suspending 'all payments immediately' to the Palestinians following the weekend Hamas attack on Israel.
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