Ten Democratic and Republican US senators have said they were open to suspending the implementation of new sanctions on Iran but only if Tehran takes significant steps to slow its nuclear programme.
Negotiations about Iran's nuclear programme are scheduled to start in Geneva on Tuesday and will be the first since the June election of President Hassan Rouhani, a relative moderate who wants to thaw Iran's icy relations with the West to secure the removal of punitive sanctions that have hobbled its oil-based economy.
In a letter to President Barack Obama on Monday, the 10 senators said the US and other countries should consider a "suspension-for-suspension" initial agreement, in which Iran would suspend uranium enrichment and Washington would suspend the implementation of new sanctions.
However, Iran is not expected to offer to suspend enrichment during the talks.
The letter, which was sent to Obama on Friday and released on Monday, was written by six Democrats and four Republicans.
They said they supported the negotiations but wanted confidence-building actions from Iran before they would support backing away from a new set of even stricter sanctions on Iran now making their way through Congress.
The senators said they wanted Tehran's full cooperation with the International Atomic Energy Agency, fulfillment of promises under the Nuclear Nonproliferation Treaty and implementation of all UN Security Council resolutions on its nuclear weapons programme, including immediate suspension of all enrichment.
"If the Iranian government takes these steps in a verifiable and transparent manner, we are willing to match Iran's good-faith actions by suspending the implementation of the next round of sanctions currently under consideration by the Congress," they said.
They also reaffirmed that "a credible military threat" remains on the table and said current sanctions must be maintained aggressively.
The senators signing the letter included Democrats Robert Menendez, chairman of the Senate Foreign Relations Committee, and Charles Schumer, the No. 3 Democrat in the Senate, as well as Republicans John McCain and Lindsey Graham, two of their party's most influential foreign policy voices.
Western nations believe Iran's uranium enrichment programme is meant to achieve a nuclear arms capability. Tehran denies this, saying it wants only to generate electricity and carry out medical research.