More than 10 Britons are feared dead or missing in Israel, Sky News understands.
It comes after Briton Corporal Nathanel Young, 20, who was serving in the Israeli army, was killed in an attack by Hamas militants.
Israeli forces are currently fighting Hamas in two locations inside Israel, after the country formally declared war on the militant group.
At least 700 people have reportedly been killed in Israel and 560 have died in Gaza as Israel retaliated with airstrikes in an operation which has been dubbed “Swords of Iron”.
In a post on Facebook, Mr Young’s family said they are “heartbroken”, adding: “Our little brother Nathanel Young was tragically killed on the Gaza Border yesterday.”
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A second British man in Israel is also missing from the area of the attacks and is feared dead.
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Dan Darlington is originally from the UK but was living in Germany and had been visiting Israel.
A third, Jake Marlowe, 26, has been “missing near Gaza” since Saturday morning after Hamas attacked a music festival in southern Israel, the country’s embassy in the UK told Sky News on Monday.
Matthew Miller, a US state department spokesperson, told CNN that nine Americans have also been killed in Israel.
“We extend our deepest condolences to the victims and to the families of all those affected, and wish those injured a speedy recovery,” he said.
Three US citizens are unaccounted for.
Mr Miller said the US is working with Israeli counterparts to determine their whereabouts.
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