However Boris Johnson has said the UK is reluctant to help enforce that because it could mean shooting down Russian planes, and could result in a war with Vladimir Putin's regime.
US Secretary of State Antony Blinken, who's in Brussels for a meeting of NATO foreign ministers, says the alliance "seeks no conflict".
But he also told reporters: "If conflict comes to us, we are ready for it."
Home Secretary Priti Patel is in Poland meeting refugees who've fled Ukraine.
She's launching a scheme which makes it easier for them to come to Britain.
Deputy Prime Minister and Esher MP, Dominic Raab, insists the UK will seize the property of Russian oligarchs if there's a legal basis to do so.