President Barack Obama became the first world leader to congratulate David Cameon on his new turn as Britain's Prime Minister.
Just hours after the 43-year-old became Britain's youngest prime minister in nearly 200 years, Mr Obama called from the White House on Tuesday evening.
The two leaders discussed Iran and Afghanistan, and are already planning a meeting in Washington over the summer.
Mr Obama said he told the new PM that the US has no closer friend and ally than the United Kingdom.
"I reiterated my deep and personal commitment to the special relationship between our two countries," Barack said in a statement.
He also prasied Gordon Brown, who he said "provided strong leadership during challenging times, and I have been grateful for his partnership."