US senators have urged President Barack Obama to impose tougher sanctions on Iran, after new President Hassan Rouhani called on the West to abandon the "language of sanctions".
Seventy-six senators said in a letter published on Monday that Washington must stress its military option.
"Mr President, we urge you to bring a renewed sense of urgency to the process," said the letter, authored by Robert Menendez, the Democratic chairman of the Senate Foreign Relations Committee, and Republican Senator Lindsey Graham.
Iran needs to understand that the time for diplomacy is nearing its end.
"We need to understand quickly whether Tehran is at last ready to negotiate seriously" on its nuclear programme, it said.
"Iran needs to understand that the time for diplomacy is nearing its end. We implore you to demand immediate serious moves on Iran's part."
Another 74 senators signed the letter, which was sent to the White House on Friday, two days before Rouhani was sworn in in Tehran.
The reputed moderate once served as Iran's nuclear negotiator, and the West is hoping that he will take a more constructive approach in long-running talks on Tehran's controversial nuclear drive.
Despite Iranian denials, the West is convinced Tehran is pursuing a nuclear bomb.
Rouhani, a moderate cleric who won a landslide victory in the June 14 presidential elections, said in his inaugural address that the only way to interact with Iran is through dialogue.
After his speech, the US said it was ready to cooperate with his government if it were serious about engagement.
"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 programme," White House spokesman Jay Carney said in a statement.
The statement added that if the new government would engage in serious discussions to find a peaceful solution to this issue, “it will find a willing partner in the US”.