Seventeen British nationals, including children, are feared dead or missing after the Hamas attack on Israel on Saturday, an official UK source has told the OceanNewsUK.
The previous estimate for the number of dead or missing was put at "more than 10".
The death toll in Israel has reached 1,200, with more than 900 people killed by Israeli air strikes on Gaza.
The deaths of Nathanel Young and Bernard Cowan have been confirmed.
Mr Young was a 20-year-old who attended JFS School, a Jewish school in North London, and was serving in the military in Israel.
The school's headteacher, David Moody, said the school's community was "devastated" and "heartbroken" at the news of his death.
He added: "Nathanel is fondly remembered within the school and we think of him with nothing but love."
Mr Young's funeral, held at Israel's national cemetery Mount Herzl, was interrupted after loud bangs were heard over Jerusalem.
More than 1,000 people turned out and listened as Mr Young's younger brother Elliot paid tribute to him.
But when his sister started to remember him, an emergency siren pierced the tranquillity and prompted mourners to throw themselves to the ground, taking cover under trees and between gravestones.
Bernard Cowan grew up in Glasgow before settling in Israel with his wife and three children.
His family said in a statement: "We are grieving the loss of our son and brother, Bernard Cowan, who was horrifically murdered on Saturday during the surprise terrorist attack on Israel by Hamas.
"We ask for privacy at this time while we process this huge loss to our family, both at home and in Israel, and to the Jewish community in Glasgow where he will be sorely missed."
The family of Daniel Darlington have also said they believe he is among those killed.
In a statement posted on Instagram, his sister referred to him as Danny and her "baby brother". She said he was killed at the Nir Oz kibbutz alongside a friend.
Jake Marlowe was working as a security guard at the Supernova music festival, where 260 people were killed when it was stormed by militants.
The 26-year-old has been reported as missing after the attack, which took place at the Re'im kibbutz around 3.7 miles (6km) from the Gaza barrier.
He was also a former pupil of JFS in North London.
Head teacher David Moody said "we have seen reports that Jake Marlowe is missing and we all pray that he is found soon".
Speaking to OceanNewsUK on Wednesday, Defence Secretary Grant Shapps said the UK is offering "moral" as well as "practical support" to Israel.
He said he had spoken to ministers in the Israeli government.
"We have asked the Israeli government to let us know what they need. And again, we continue to talk to them about that."
