Gordon Brown has said Britain's "most important" relationship is with the US, in his first major foreign policy speech since becoming prime minister.
He warned that he had "no truck with anti-Americanism" and said the EU should strengthen ties with the US.
Mr Brown also called for more "hard-head internationalism" in peacekeeping, aid and reconstruction.
In the speech at the Lord Mayor of London's banquet, he also warned Iran over its "nuclear ambitions".
Mr Brown told the audience at Guildhall the UK had to work with "all those who share our vision of the future", including Nato, the UN, the EU and the US.
He said: "It is no secret that I am a life-long admirer of America.
"I have no truck with anti-Americanism in Britain or elsewhere in Europe and I believe that our ties with America - founded on values we share - constitute our most important bilateral relationship.
"And it is good for Britain, for Europe and for the wider world that today France and Germany and the European Union are building stronger relationships with America."
The speech followed reports that Mr Brown was keen not to appear as close to President Bush as his predecessor, Tony Blair.
BBC US correspondent Jonathan Beale said there had been a "distinct autumn chill" in UK-US relations.
He added: "The question is can relations with Gordon Brown be as close as with Tony Blair?"
In his speech, Mr Brown signalled support for the US stance on Iran's "nuclear ambitions", saying Tehran had a choice between "confrontation" or a "transformed relationship with the world".
He added: "Iran should be in no doubt about our seriousness of purpose."
Shadow foreign secretary William Hague said: "We have called for many months for international sanctions targeted at investment in Iranian oil and gas, and its financial sector.
"The prime minister has listened and has finally come round to our way of thinking."
He added: "We remain deeply concerned that neither the UN, nor European nations, have taken any action against Iran in months.
"Far more needs to be done to put pressure on the regime in Tehran: a UN embargo on new arms sales to Iran, more effective steps against those involved in Iran's nuclear programme, action to target the Iranian Revolutionary Guard Corps, and restrictions on export credits to Iran."
Watch the Prime Minister's Guildhall speech in Briefings on BBC Parliament at 2100 GMT this Saturday