The US has said it sees "troubling developments" in Iran's nuclear programme and called on the country's new president to take concrete steps soon to ease concerns about Iran's aims.
Iranian President Hassan Rouhani, a relative moderate, has pledged to improve Iran's relations with world powers to help ease stringent international sanctions imposed on the Islamic Republic over its atomic activities.
Joseph MacManus, US ambassador, said on Wednesday that the US was ready to work with the new Iranian government "to reach a diplomatic solution that will fully address the international community's concerns" about Iran's nuclear programme.
"We are hopeful that the Rouhani administration will live up to its assurances of transparency and cooperation by taking concrete steps over the next several months," he told the 35-nation board of the International Atomic Energy Agency (IAEA) in Vienna, Austria, according to a copy of his speech.
"We are mindful of this unique moment, and urge Iran to take the steps necessary to allay international concerns about the nature of its nuclear programme."
Iran says it is enriching uranium only for civilian energy and medicine.
The West suspects the programme is covertly oriented towards developing the capability to make nuclear weapons.
MacManus noted that Iran had further expanded its uranium enrichment capacity by continuing to install both advanced and first-generation centrifuges, calling these steps "concerning escalations of an already prohibited activity".
Iran is also making further progress in the construction of a reactor, Arak, that can yield plutonium for bombs, including putting the reactor vessel in place and beginning to make fuel, he said.
"All of these are troubling developments," he said.