The inauguration of the new president of Iran, Hassan Rouhani, presents "an opportunity" for Tehran to resolve concerns over its nuclear programme and better engage with the west, the Obama administration said on Sunday.

In a statement marking the formal handing over of power to the moderate cleric from his hardline predecessor, Mahmoud Ahmadinejad, the White House congratulated the Iranian people for "making their voices heard" and calling for "change". It added that should the new government in Iran move towards meeting its "international obligations", then it would find a "willing partner in the United States".

But the apparent olive branch comes amid hawkish calls in Washington for tougher sanctions on Tehran and the possibility of military action if no resolution is found. In a letter sent to President Barack Obama, 76 senators demanded tougher economic punishment for Iran until the Islamic republic scales back its nuclear ambitions. It also urged Obama to keep all options on the table, while keeping the door open to diplomacy.

"Until we see a significant slowdown of Iran's nuclear activities, we believe our nation must toughen sanctions and reinforce the credibility of our option to use military force at the same time as we fully explore a diplomatic solution to our dispute with Iran," the letter states, according to a version seen by the Associated Press.

The letter comes just days after the House of Representatives overwhelmingly passed new restrictions on Iran's oil sector and its mining and construction industries. Senators are expected to take up the same package in September.

The stance from Congress is at odds with that of the Obama administration, which has pursued a softer line on Iran as it transitions the presidency. Rouhani has pledged to follow a "path of moderation" and promised greater openness over Iran's nuclear program. The White House appears willing to give the president a chance to put those pledges into action.

The White House statement said: "We note that President Rouhani recognised his election represented a call by the Iranian people for change, and we hope the new Iranian government will heed the will of the voters by making choices that will lead to a better life for the Iranian people. The inauguration of President Rouhani presents an opportunity for Iran to act quickly to resolve the international community's deep concerns over Iran's nuclear program."

The US has long believed that Iran has been working to develop nuclear weapons. Iran insists its program is for peaceful energy and research purposes only.

The last round of talks between Iran and the wider international community over the issue broke down in April, with seemingly little progress. At that point, US secretary of state John Kerry hinted that negotiations could be heading towards a deadline, noting that the talks were not an "interminable process". But the election of Rouhani has seemingly raised hopes of diplomatic solution to the impasse.

• This article was amended on 20 September 2013. It originally called the Iranian president Hasan Rouhani. It should be Hassan Rouhani. This has been corrected.