The United States doubts that European diplomacy will succeed in keeping Iran from developing nuclear weapons after Iran's election of a new president.
Solana said any unilateral military action would be counterproductive
"We have reason to be sceptical," White House spokesman Scott McClellan said before a visit by German Chancellor Gerhard Schroeder to Washington.
EU foreign policy chief Javier Solana said earlier the union saw no reason to change its policy towards Tehran.
UN inspectors were at work in Iran on Monday, Iranian officials said.
The International Atomic Energy Agency would neither confirm nor deny the claim, saying it did not comment on where its inspectors were at work at any given moment.
Mr Solana said Germany, France and Britain's effort to prevent Iran from developing nuclear weapons would continue.
"We don't have any reason to change at this time," he told reporters.
President-elect Mahmoud Ahmadinejad has announced Iran will continue pursuing a peaceful nuclear programme.
On Sunday, a spokesman for Mr Solana was reported as saying that it was "to soon to say" what would happen next.
"For EU-Iran relations to reach their full potential, progress is necessary on a number of issues, including human rights, non-proliferation, counter-terrorism and the Middle East process," he added.
European officials have voiced concern over Mr Ahmadinejad's statements, despite his pledge to continue nuclear talks with the EU.
"From the new President Ahmadinejad we are waiting for clear words on human rights and the nuclear issue," EU justice commissioner Franco Frattini told Italy's La Repubblica newspaper in an interview published on Sunday.
"But if the replies are negative, the European Union will have no choice but to freeze the dialogue with Iran."
Mr Frattini went on to say that the "litmus test" for the new Iranian president would be whether he was willing to take part in the next round of talks scheduled for September.
In his first news conference after his election, Mr Ahmadinejad said Iran had "a right to achieve scientific development in all fields".
He added that the country needed "peaceful nuclear technology" for medical and agricultural purposes as well as for its scientific progress.
He also underlined that Iran did not have "significant need" for ties with the US.
Foreign ministers from the three EU countries leading the talks, Germany's Joschka Fischer, Britain's Jack Straw and France's Philippe Douste-Blazy, have all expressed their hope that discussions would continue.
"Economic cooperation also rests on how successfully Iran can secure international trust and further open itself," Mr Fischer told the German daily Bild.