It comes as Ukraine’s President Volodymyr Zelensky, in a virtual address to a UK-led defence grouping meeting, called for “collective efforts” to defeat Russian President Vladimir Putin as the war drags into another winter.
The Prime Minister was on the Baltic island of Gotland on Friday for a gathering of the 10-strong Joint Expeditionary Force, where the war in Ukraine and the escalating crisis in Israel and Gaza topped the agenda.
The invasion of Ukraine has focused minds in northern Europe about the threat posed by Mr Putin, with a fresh emphasis placed on security co-operation in the region.
Mr Sunak, who spent Thursday night aboard a Royal Navy frigate in Gotland, began his address to fellow leaders with a pledge to support both Ukraine and Israel.
“I think it’s important for us to say that we’re all outraged by the terrorist attacks that have happened in Israel.
“As you will all agree that we will stand with Israel at this moment. I’ve offered Prime Minister (Benjamin) Netanyahu my full support.
“Terrorism will not, must not, prevail, in Ukraine, in Europe, in anywhere else.”
Mr Zelensky joined proceedings virtually, as leaders stressed their continued backing for Kyiv.
“Now is the time for collective efforts. It does not mean that capabilities of one nation are too weak,” Mr Zelensky said.
“It means that the task we face is too big. We need reliable geo-political stability in Europe and more broadly in the whole Euro-Atlantic space. So we need new collective forums of collaboration and strengthening of existing ones.
“Now before winter comes we should all focus on hybrid terrorist threats from Russia. We predict that this winter Russia will try to repeat its terror tactics.”
Five female and five male leaders gathered on the Baltic island, as Mr Sunak held a series of meetings with his Swedish, Norwegian and Danish counterparts.
Mr Sunak met Swedish prime minister Ulf Kristersson aboard HMS Diamond ahead of the formal gathering, something the British leader described as a “symbol” of the co-operation between the two countries.
The two men signed a new strategic partnership agreement.
Mr Sunak described the Joint Expeditionary Force as the “first line of defence”, as he praised progress on defence co-operation.
“We’ve done more in the last 18 months than we’ve done in almost a decade before,” he said.
“I think that speaks to the energy which we put into this gathering but also the urgency that the situation demands.”
The gathering came amid concerns about the possible sabotage of an undersea gas pipeline between Finland and Estonia.
Downing Street said Mr Sunak, in his meeting with Norwegian leader Jonas Gahr Store, discussed the “vital need to reinforce security around subsea infrastructure”.
The Government said that more than 20,000 British troops would be deployed across northern Europe next year, amid stepped-up in security in the region.