The United States would like to see "a very strong United Kingdom staying in a strong EU", secretary of state John Kerry has said.
Mr Kerry said it was "profoundly" in America's interests that the UK voted to remain in the union in the upcoming referendum.
David Cameron's renegotiation drive is approaching its moment of truth at a Brussels summit next week.
Speaking at a security conference in Munich, Mr Kerry said: "I want to express the confidence of President Obama and all of us in America that, just as it has so many times before, Europe is going to emerge stronger than ever, provided it stays united and builds common responses to these challenges.
"Now obviously, the United States has a profound interest in your success as we do in a very strong United Kingdom staying in a strong EU."