Prime Minister Gordon Brown stressed the importance of ties with the United States and outlined foreign policy priorities Monday, pledging to press for tough new sanctions on Iran over its nuclear program.

Brown, who succeeded the staunchly pro-American Tony Blair as prime minister in June, has been closely scrutinized for any sign he wants to reorder the so-called "special relationship" between the two allies. Last month, he said Britain will halve its remaining force of 5,000 troops in Iraq by next spring.

In a speech before the annual Lord Mayor's banquet, however, the prime minister said the world was a better place when the U.S. and Britain acted together.

"It is no secret that I am a lifelong admirer of America. I have no truck with anti-Americanism in Britain or elsewhere in Europe," Brown said. "I believe that our ties with America — founded on values we share — constitute our most important bilateral relationship."

Brown used his speech to warn Tehran that unless pending reports on its contested nuclear program show signs of progress, he will push the European Union and United Nations for tighter sanctions.

"Iran should be in no doubt about the seriousness of our purpose," Brown told the annual Lord Mayor's banquet on Monday.

Brown proposed new restrictions on investment in Iran's oil, natural gas and finance sectors during the sweeping foreign policy speech.

Mohamed ElBaradei, the head of the U.N's International Atomic Energy Agency and Javier Solana, the European Union's foreign policy chief, will report soon on Iran's progress toward disclosing full details of its past nuclear activities and on suspending uranium enrichment — a possible pathway to nuclear arms.

Britain, the United States and other allies claim Iran's nuclear program is a cover for weapons production. Iran insists it is attempting to produce an alternate source of energy.

Blair's unpopularity in the waning days of his third term was widely blamed on his personal alliance with President Bush, and statements by some British officials have suggested that Brown might seek to distance himself from Bush. Junior Foreign Affairs Minister Mark Malloch-Brown said earlier this year the two leaders would not be "joined at the hip."

But in his speech, Brown welcomed the pro-American tilt of new French President Nicholas Sarkozy and German Chancellor Angela Merkel.

He said it was "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."