Condoleezza Rice has insisted that attacking Iran is not on the US agenda "at this point in time".
Ms Rice will also meet Israeli and Palestinian leaders
She was speaking in London following meetings with Tony Blair and Jack Straw, on her first overseas trip as US secretary of state.
She said the US would use diplomacy to deal with Iran's nuclear programme.
But she attacked its human rights record and claimed it was harming prospects for peace in the Middle East by supporting terrorism.
Ms Rice described the half hour meeting with Mr Straw and Mr Blair as "productive" and hailed the strength of the US/UK friendship, saying America had "no better friend and no better ally".
Asked if she envisaged circumstances in which the US would attack Iran, she said: "The question is simply not on the agenda at this point in time."
She added: "We have many diplomatic tools still at our disposal and we intend to pursue them fully."
But she said the Iranian people "deserved better", and condemned the regime's "abysmal human rights record".
'Supporter' of terrorism
She told BBC's Breakfast with Frost that America's primary goal is "to deal with Iran's destabilising influence" on the international scene which is "one of the most important barriers" to the Israel-Palestinian peace process.
Iran is "the key supporter" of rejectionist groups like Hezbollah, she said.
"We need unity of purpose, and unity of message to Iran to stop those activities."
Attention has been focused on Iran's civilian nuclear programme amid fears that the Tehran regime is trying to build a nuclear bomb.
Mr Straw hailed efforts by the UK, Germany and France to secure a diplomatic resolution to the crisis, insisting that Washington had been "very supportive of the process".
He later told BBC's the World At One: "As Condoleezza Rice and indeed President Bush have said, they are backing the diplomacy which is being led by France, Germany and the UK."
As a result of agreements reached, all of Iran's uranium enrichment and related activities, apart from some "very limited compliance", have been suspended, he said.
On Iraq, Mr Straw highlighted the success of the country's recent elections as helping heal world divisions over the war.
Ms Rice's week-long tour of Europe and the Middle East includes talks with the Israeli and Palestinian leaders.
She also confirmed her attendance at a Palestinian conference in London next month.
During the rest of the trip, Ms Rice is expected to give a staunch defence of President George Bush's stated aim of spreading freedom and democracy around the world in what is seen as a bid to mend relations with nationals opposed to the Iraq war.
After leaving London, she flew to Germany for talks with Chancellor Gerhard Schroeder.
Her whistle-stop tour is also scheduled to include stops in Belgium, Luxembourg, Turkey, Italy and Poland.
Ms Rice will round off the trip by making a major speech on US-EU relations in Paris.