She is British. She is a Jewish Brit born & raised in London. She has a common modern London accent when she speaks in interviews. But in her songs, she is putting on a different accent, just for the song, and this accent is probably influenced by American pop music culture, but it doesn't sound particularly like British or American English to me, it just sounds like a quirky modern pop song. She is singing in a particular way that the pop industry wants her to sing, and her normal voice is quite different when you listen to her speaking in an interview.