The US State Department today called Iran's promise to freeze all activity related to nuclear enrichment useful but said the suspension had to be verified.
In his first formal comment on the agreement between Iran and Britain, Germany, France and the European Union, State Department spokesman sounded a note of optimism along with Washington's typical skepticism about Iranian promises.
"This is a useful step," Mr Boucher told reporters. "It's better to have somebody agree to something than not agree to something. But it doesn't really make a difference until it's implemented and verified, and that's what counts."
The United States accuses Iran of seeking to develop nuclear weapons and wants the matter to be addressed by the UN Security Council, which could impose sanctions. Tehran says its nuclear program is aimed at generating electricity.
Under the deal unveiled yesterday, Tehran agreed to freeze all activity related to enrichment, including making equipment and processing materials, from November 22 in return for talks on peaceful nuclear cooperation and resumed trade and aid talks.
Uranium enrichment is a process that produces fuel which can be used for power generation or atomic bombs.
The deal is likely to undercut support for the US push to have Iran's case reported to the Security Council at the International Atomic Energy Agency's Nov. 25 board meeting.
A senior State Department official who spoke on condition he not be named told reporters there may be little backing for this if the IAEA says the agreement is being carried out.
"(If so) probably there will be many people who want to give it more time or keep them under review," he said.