After chairing an emergency meeting of Cobra on Monday, the prime minister said the UK stood “poised” to help Israel militarily if it asked for assistance.
Downing Street also said the Government was not ruling out evacuating some British citizens from Israel.
The prime ministers official spokesman said: “It’s a fast-moving situation that the Foreign Office are keeping under review.
"We are contacting relevant groups and nationals where required.”
Around 50,000 to 60,000 British nationals are believed to be in either Israel and Gaza, the Government estimates.
Meanwhile, at least 1000 people have reportedly been killed in Israel over the last three days after Hamas terrorists launched an unprecedented attack on the Jewish state.
Speaking to broadcasters during a visit to east Staffordshire, Sunak added: "I know there will be families who are anxious about their loved ones in the region.
"I want to reassure them we're doing everything we can, working very closely with the Israeli authorities to establish what is happening on the ground, provide support to people where it is needed."