US secretary of state John Kerry has voiced support for Britain staying in the EU, saying the continent needs to be "united".
Mr Kerry said it was "profoundly" in America's interests that the UK voted to remain in the union in the looming referendum.
The comments come as David Cameron's renegotiation drive approaches its moment of truth at a Brussels summit next week.
In his last big speech before the gathering of leaders, the Prime Minister told an audience in Hamburg on Friday night that Europe had to "stand together" against threats such as Islamic State (IS) and Russian aggression.
He also appealed for Germany's help in finalising his package of reforms, stressing the countries' shared interests and values.
Speaking at a security conference in Munich, Mr Kerry said the EU was facing a number of challenges including Brexit.
"Here again however, 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," he said.
"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."
There are reports that Mr Obama, who has previously voiced support for British membership of the EU, is preparing to make a "big, public reach-out" to voters once the referendum campaign begins.
Senate foreign relations committee chairman Bob Corker discussed the tactic openly with witnesses during an evidence session, saying he "knew" that was Mr Obama's intention.
Leave.EU spokesman Jack Montgomery said: "It might be convenient for John Kerry, who has repeatedly declined to support the UK in the Falklands, for us to be in the EU, but that doesn't mean it's good for us.
"Imagine if Kerry proposed a pan-American union in which an unelected commission would control United States immigration policy, trade policy and regulations, among a host of other important matters. He would be run out of Washington DC on a rail."