Mr Sunak is set to meet Crown Prince Mohammed bin Salman on Thursday evening as he urges leaders in the region not to allow the Israeli-Hamas battle escalate into a wider conflict.
The Prime Minister tweeted: “As an international community, we must not let Hamas’s terror attack become a catalyst for a terrible humanitarian crisis in Gaza.
“We will work together to ensure regional stability and prevent a dangerous escalation.”
He travelled from Tel Aviv after meetings with Israel’s prime minister Benjamin Netanyahu and president Isaac Herzog.
Mr Netanyahu told Mr Sunak that he hoped for the UK’s “continuous support” in his country’s “long war” as it fought back against Hamas following the Palestinian militant group’s deadly and unprecedented assault on Israel on October 7.
The Israeli premier said his forces were fighting against the “worst monsters on the planet” during the “world’s darkest hour” as he warned there would be “ups and downs” during the war.
Mr Sunak, speaking at a joint press conference after discussions with his counterpart lasting about two hours, said he was “proud” to stand with Israel and that the UK Government “wants you to win”.
Israel defence minister Yoav Gallant has told ground troops on the Gaza border to “be ready” to enter the Occupied Palestinian Territory, but he did not say when the invasion would start.
“Whoever sees Gaza from afar now will see it from the inside,” he told Israeli infantry soldiers.
Asked what Mr Sunak made of Mr Netanyahu’s assessment that Israel was facing a “long war”, a spokeswoman for the Prime Minister said: “It is not for us to comment on the detail of the Israeli approach.”
Downing Street has updated the number of Britons killed in Hamas’s assault on Israel since the Prime Minister’s arrival in Saudi.
A spokeswoman for the Prime Minister said: “We can now confirm that at least nine British nationals were tragically killed in last week’s terrorist attacks.
“And a further seven British nationals are missing, some of whom are feared to be among the dead or kidnapped.”
In a statement to the Commons on Monday, the Prime Minister said six British nationals had died, with a further 10 missing.
The PA news agency understands that Yonatan Rapoport was killed during the Hamas ambush which left hundreds of Israelis dead.
Mr Rapoport, known as Yoni, was reportedly one of those murdered in the attack on Kibbutz Be’eri.
He had two children, Yosefi and Aluma, and, as a Manchester United fan, had planned to take his son to Old Trafford next month for his first game, the BBC reported.
The UK Government has not confirmed how many Britons are among the hostages held by Hamas, although at least two are known to be being kept captive, with Mr Sunak meeting with their families while in Tel Aviv.
The British leader thanked the Israeli government for its efforts in helping to secure the release of UK citizens being held hostage. About 200 people in total are said to have been snatched in the Hamas raids.
Mr Sunak has welcomed Mr Netanyahu’s decision, after lobbying by US president Joe Biden, for Israel to start letting aid into Gaza.
It marks a potential break in a punishing siege on the territory after Israel’s retaliation on Hamas saw it block access for water, food, electricity and other essential supplies to the 25-mile strip, while also carrying out strikes.
The Prime Minister said the UK would seek to get “more support to people as quickly as we can” as he reiterated that Palestinians living under Hamas rule in Gaza were also victims.
Israel continued to pound locations across Gaza on the day of Mr Sunak’s visit to Tel Aviv, including firing on parts of the southern strip that had been declared safe zones.
A residential building in Khan Younis, a city in southern Gaza where hundreds of thousands of Palestinians had sought shelter, was among the places hit.
Medical personnel said they had received at least 12 dead and 40 wounded. Thousands in Gaza have died since retaliatory Israeli strikes started in the aftermath of Hamas’s incursion.
Many among Gaza’s 2.3 million residents have cut down to one meal a day and have been left to drink dirty water amid dwindling supplies.
The UK Government is being urged to lobby for a ceasefire by both British political leaders and charities working in the region.
Scotland’s First Minister, Humza Yousaf has called on Foreign Secretary James Cleverly, who is on his own diplomatic blitz across Egypt, Qatar and Turkey, to press for a resolution that could help those UK citizens “trapped in Gaza”.
Mr Yousaf, whose mother and father-in-law are stuck in the territory, said the lack of focus on those unable to escape the bombardment in the 25-mile strip was “shameful”.
In a post on X, formerly Twitter, the SNP leader said: “They’re trapped in a war zone and need you, Foreign Secretary, to demand a ceasefire, and the Rafah crossing opened to let them out.”
Katy Chakrabortty, Oxfam’s head of advocacy, said: “The Prime Minister is right to call for the safe return of all hostages in Gaza but his failure to call for a ceasefire is wrong.
“Wanting Israel to ‘win’ is not a helpful statement. Nobody wins when the scale of human suffering, devastation and pain is so vast and relentless. We need all parties to commit to a ceasefire and for full, unfettered humanitarian access to Gaza now.”
Mr Sunak is expected to stay overnight in Saudi Arabi following talks with the Crown Prince. No 10 is yet to confirm whether he will visit another Middle East capital before wrapping up his two-day trip on Friday.