Sadiq Khan's election as mayor of London has been hailed as an "historic development" by the White House.
Khan's election last week made him the first Muslim mayor of a Western capital city and drew global attention.
The mayor began work in City Hall today and was greeted by cheering Labour MPs at a meeting of the Parliamentary Labour Party this evening following his victory over Conservative Zac Goldsmith.
Speaking on Monday, President Obama's press secretary Josh Earnest said: "This is obviously an historic development for an historic city. Obviously the political debate inside of the UK and in London is obviously charged with a lot of different issues.
"This outcome reflects the will of the people of London and I certainly wouldn’t second guess them from here. There is no denying the historical significance of Mr Khan’s election."
He added: "I don't know enough about the election to offer my own analysis about what may have motivated the voters of London. But obviously this is an historic election."
Hillary Clinton, the frontrunner to be the Democratic nominee for president, has also congratulated the new Labour mayor.