Sir Keir made the remarks as he travelled to Canada for the G7 summit, where he said the weekend's"intense" developments would be further discussed.
"We've already been moving assets to the region, including jets, and that is for contingency support across the region," he told reporters.
"Our constant message is de-escalate, and therefore everything we're doing, all discussions we're having are to do with de-escalation."
The prime minister would not be drawn on whether the UK would be involved in defending Israel.
"I had a good and constructive discussion with Prime Minister [Benjamin] Netanyahu yesterday, and that included discussions about the safety and security of Israel, as you would expect, between two allies," he said.
He reiterated that the UK had"long-standing concerns" about Iran's nuclear programme and recognised Israel's right to self-defence.
Iran has threatened to target UK, French and US military bases in the region if they offer Israel help to stop Tehran's strikes.
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Sir Keir spoke to Saudi Arabia's Crown Prince Mohammed bin Salman on Saturday afternoon, Downing Street said.
"They discussed the gravely concerning situation in the Middle East and agreed on the need to de-escalate," a spokesman said.
Iran launched waves of missile and drone attacks on Israel overnight, following Israeli strikes on Friday.
The Israeli military said it is still striking Iran, while Iran's President Masoud Pezeshkian threatened a"more severe" response if Israel continues.
Iranian state TV reported 60 people had been killed in a strike in Tehran, while in Israel, officials said three people had been killed and dozens injured.
Washington DC and Tehran were due to resume talks on Iran's nuclear programme on Sunday, but the talks have now been cancelled, mediator Oman said.
