By Sophie Walker
LONDON (Reuters) - Britain will push to curb energy and financial investment in Iran unless Tehran addresses nuclear concerns, Prime Minister Gordon Brown said on Monday in a speech that also underlined Washington's importance as an ally.
"We will lead in seeking tougher sanctions both at the U.N. and in the European Union, including on oil and gas investment and the financial sector," Brown said, unless there are positive reports this month from EU foreign policy chief Javier Solana and the United Nations nuclear watchdog.
"Iran should be in no doubt about our seriousness of purpose," he said in his first major foreign policy speech since becoming premier in June.
The five permanent members of the U.N. Security Council -- Britain, the United States, France, Russia and China -- plus Germany, agreed earlier this month to proceed with a third round of sanctions against Iran.
Iran has refused to halt uranium enrichment after two previous U.N. sanctions resolutions and denies the West's allegations that it wants to make atomic bombs, saying its program is for peaceful power generation.
The United States has not ruled out military action against Iran. Brown has urged a diplomatic solution, while also saying he will not rule out anything.
In his speech, Brown emphasized he was a "life long admirer of America" and added: "I believe that our ties with America -- founded on values we share -- constitute our most important bilateral relationship."
Former Prime Minister Tony Blair's close relationship with President George W. Bush, derided by some as too close, contributed to the popularity slump that hastened his departure from office after 10 years.
Since taking over, Brown has sought to distance himself from much of Blair's foreign policy program and has made it clear Britain plans to start withdrawing troops from Iraq as soon as possible -- policies which are reported to have irritated some policy advisers at the White House.
Some British analysts have suggested the new prime minister, while seeking to dissociate himself from Iraq, still wants to assure the Bush administration of his support and has chosen to do so via the prism of the Iran negotiations.
"Brown doesn't want to be Blair. But Iran is one place where he can signal to the Americans that they're still on the same side," Ali Ansari, director of the Institute of Iranian Studies at the University of St Andrews in Scotland, said last week.
Brown said Britain would continue to work with international institutions such as the United Nations, IMF and World Bank, but stressed the institutions need to reform.
(Editing by Matthew Jones)