The US State Department has said that Hassan Rouhini's inauguration is a chance for Iran to "act quickly" on nuclear concerns but added that, "the ball is in their court" to take credible action.
Russia says Iran's new president should be supported in his stance on nuclear talks. Russia said it "absolutely agreed" with Iranian leader Hassan Rouhini in saying that disputes over Iran's nuclear programme must be solved by dialogue, not ultimatums.
Speaking during a visit to Rome, Russian Foreign Minister Sergei Lavrov said: "Now it is critical to support the constructive approach of the Iranian leadership."
The White House said it hoped Hassan Rouhani would "heed the will" of voters and make choices that would "lead to a better life" for Iranians.
Press secretary Jay Carney said the new president's inauguration gave Iran a chance "to act quickly" to address international concerns about the country's disputed nuclear program.
The White House added that Rouhani would find "a willing partner" in the US if his government decided "to engage substantively and seriously" in honouring its international obligations.
Iran's new president Hassan Rouhani welcomed senior politicians from various countries who were invited to his inauguration ceremony in Tehran.
Those attending included Syrian Prime Minister Wael Nader al-Halqi and Vice President of Iraq, Khodair al-Khozaei.
The president of Kazakhstan, Nursultan Nazarbayev, and the president of Tajikistan, Emomalii Rahmon also attended.
Rouhani also welcomed the envoys of Cuba and Nicaragua, two Latin American countries with strong diplomatic ties to Iran.
The inauguration of Iran's new president Hassan Rouhani will take place today a day after he assumed office to succeed Mahmoud Ahmadinejad.
Supreme Leader Ayatollah Ali Khamenei yesterday endorsed Mr Rouhani's election win in a statement read out to political, religious and military grandees assembled at a religious site in Tehran.
The 64-year-old, who will assume office after taking an oath in Iran's parliament later, arrives in power at the start of 12 months which are seen as make or break for the international effort to prevent Iran from acquiring nuclear weapons.
The resounding victory of the politically relatively moderate leader in June's election raised hopes of negotiating an end to the nuclear dispute and an easing of the sanctions that have hit the OPEC country's oil exports.
Hassan Rouhani has taken office as Iran's president, succeeding Mahmoud Ahmadinejad, after Supreme Leader Ayatollah Ali Khamenei endorsed him, state TV said.
Rouhani, a relatively moderate cleric who won an election in June, will take the oath of office in a ceremony in Iran's parliament on Sunday.
Iran's president-elect Hassan Rouhani called Israel a "wound" that must be removed, Iranian media reported.
Speaking two days ahead of his inauguration, he is reported as saying: "The Zionist regime is a wound that has sat on the body of the Muslim world for years and needs to be removed."