Messi enters the final six months of his contract on January 1, allowing him to hold discussions with foreign clubs ahead of a move later in 2021.
Speaking to La Sexta, Messi said: "I've always had the dream of playing in another league, in United States.
"Maybe it will happen, doesn't have to be right away. Today I am just focused on these next six months."
Should Man City make a fresh bid for the playmaker, they are able to offer Messi an opportunity to represent New York City FC later in his career.