Two British teenage sisters are among those still missing after Hamas militants stormed their home in Israel.
Noiya, 16, and Yahel, 13, disappeared from Be’eri kibbutz after it was raided.
Their mother, Lianne, was killed and their father, Eli, is still missing.
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It comes after Rishi Sunak told MPs on Monday at least six Britons were killed and a further 10 are missing in the “pogrom” on 7 October and called for the immediate release of the around 200 hostages taken by Hamas.
Raz, 52, the father’s brother-in-law, is reported to have called on the British government to “take some action” or find out what happened to the family.
“We don’t know if they were kidnapped but if we come to that point we will be happy if they try to do whatever they can to bring those girls back,” he told The Times.
British-Israeli national Benjamin Trakeniski, 32, was also killed in Be’eri while trying to help residents flee Hamas gunmen, according to multiple reports.
Earlier on Monday, a Hamas official demanded 6,000 Palestinian men and women detained in Israel’s jails be released in exchange for hostages in Gaza.
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‘Pogrom’ committed by Hamas in Israel
Khaled Meshaal, head of Hamas’s diaspora office, said the hostages include high-ranking officers from the Israeli Defence Forces’ Gaza Division, which is responsible for patrolling around the Gaza Strip.
In a statement to the Commons, Mr Sunak said he recognised the Palestinian people are “victims of Hamas too” as he announced an extra £10m of aid funding.
British officials have been pressing for Egypt to open the Rafah crossing with Gaza to allow aid in and Britons, dual nationals, as well as their spouses and children, to flee.
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However, a blockade remains in place, contributing to fears that citizens could starve and hospitals would run out of electricity.
It is understood the opening of the border for foreign nationals and Palestinian refugees could be treated separately.
The prime minister raised questions about the border during a call with Egyptian president, Abdel Fattah al Sisi, last week
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So far, eight flights have brought back 500 British nationals from Israel.
Mr Sunak said the UK is working with Israel to establish the facts and support the families of the missing and the dead through their “unimaginable pain”.
“The elderly, men, women, children, babes in arms, murdered, mutilated, burned alive,” he said.
“We should call it by its name: it was a pogrom.”