A written statement from the White House called the election of moderate Hassan Rouhani "presents an opportunity for Iran to act quickly to resolve the international community's deep concerns over Iran's nuclear program."
"Should this new government choose to engage substantively and seriously to meet its international obligations and find a peaceful solution to this issue, it will find a willing partner in the United States," the statement said.
The statement did not directly congratulate Rouhani, but cheered what it concluded was a fair election that could lead to further reforms in Iran. "We again congratulate the Iranian people for making their voices heard during Iran's election," it said.
Rouhani said in his inaugural address Sunday that foreign enemies should forget the idea his election would fuel unrest in the Islamic Republic because the June election had been fully transparent and peaceful.
"Its results surprised many," Rouhani said. "Because Iran, which was imagined by ill-wishers to start experiencing domestic conflicts and a wide gap between the state and the nation as a result of sanctions and isolation, created an almost unique epic through hectic election and massive participation of the people from different walks of life."
Rouhani added the conduct of the June voting "displayed maturity of the ruling system and built increasing trust between the ruling system and the people."
Iran's semiofficial FARS news agency said nearly 250 Iranian officials and foreign dignitaries attended Sunday's inauguration.
Among the audience was Sergei Naryshkin, speaker of the Russian parliament, who later told reporters Moscow hoped to see a continuation of the current state of relations with Iran. Russia has been working closely with Iran in the development of its controversial nuclear power program, which the United States contends is actually a covert effort to build nuclear weapons, Russia's RIA Novosti news service said.
Naryshkin said he hoped to see a thaw in tensions between Iran and the West, but added: "At the same time, Russia's position on the Iran nuclear issue has not changed. We believe that Iran has the right to develop its peaceful nuclear program under the conditions of full openness and under the control of the International Atomic Energy Agency."