Seventy-six US senators have urged President Barack Obama to impose tougher sanctions on Iran, saying Washington must stress its military option even as new President Hassan Rowhani urges dialogue.
"Mr. President, we urge you to bring a renewed sense of urgency to the process," said the letter, publicized Monday by authors Robert Menendez, the Democratic chairman of the Senate Foreign Relations Committee, and Republican Senator Lindsey Graham.
"We need to understand quickly whether Tehran is at last ready to negotiate seriously" on its nuclear program, it said.
Another 74 senators signed the letter, which was sent to the White House on Friday, two days before Rowhani 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.
"Iran has used negotiations in the past to stall for time, and in any event, (supreme leader Ayatollah Ali) Khamenei is the ultimate decision-maker for Iran's nuclear program," the senators wrote to Obama.
"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."
The White House said Sunday that Iran will find the United States a "willing partner" if Rowhani is prepared for serious nuclear talks.
Last week shortly before Congress went on summer break, the House of Representatives overwhelmingly approved harsh new sanctions aimed at drastically reducing Iran's oil exports by ending purchasing exemptions to countries like China and Japan.
The recess window is being seen as a pivotal period for both the White House and Iran.
A Senate official told AFP that after the August recess the Senate Banking Committee aims to introduce legislation on further sanctions against the Islamic republic.
"Our nation must toughen sanctions and reinforce the credibility of our option to use military force," the senators wrote.
"We must be prepared to act, and Iran must see that we are prepared."
Rowhani inherits a turbulent nation that is struggling economically, largely due to crippling sanctions already on the books.
But he insisted Sunday that "if you want a proper answer, do not speak with Iran with the language of sanctions but with the language of respect."
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