World powers are preparing for for two days of talks with Iran that could decide whether the decade-old crisis over its nuclear program can be peacefully resolved.
Six world powers - the United States, Russia, China, France, Britain and Germany - hold talks with Iran on its nuclear program in Geneva on Tuesday and Wednesday.
Late Monday, the European Union's foreign policy chief Catherine Ashton, who will represent the 6 nations, said she comes to the table with "cautious optimism.''
She met with Iranian Foreign Minister Mohammad Javad Zarif, who would not speculate on the talks.
In Washington, a bipartisan group of leading U.S. senators said they are open to suspending the implementation of new sanctions on Iran but only if Tehran takes significant steps to slow its nuclear program.
In a letter sent to President Barack Obama last week and released Monday, the 10 senators said the U.S. and other countries should consider a "suspension-for-suspension" agreement, in which Iran suspends uranium enrichment and Washington suspends the implementation of new sanctions.
But Iran is not expected to offer to suspend enrichment during the talks.
A senior U.S. official, speaking on condition of anonymity, told reporters that "no one should expect a breakthrough overnight'' and described the upcoming negotiations as "very, very difficult."
The official said incremental steps are needed to build confidence and "put time on the clock" so negotiators can work through complex political and technical issues.
In a message late Sunday on his Facebook page, Zarif - his country's chief nuclear negotiator - said he is hopeful the negotiations with world powers can result in a road map toward resolving the dispute.
U.S. Secretary of State John Kerry said Sunday the window for diplomacy over Iran's nuclear program is "cracking open."
In previous rounds of negotiations, world powers called for Iran to give up its existing stockpile of uranium enriched to 20 percent purity and send it abroad. Uranium of that purity is a short technical step away from being converted to weapons-grade material.
The nuclear talks will be the first since Iranian President Hassan Rouhani took office in August, promising to lead a diplomatic effort to ease Western sanctions on his country.
His predecessor, Mahmoud Ahmadinejad, defied U.N. Security Council demands to suspend Iran's uranium enrichment program, which Western powers believe is aimed at making nuclear weapons. Iran insists its nuclear program is for peaceful purposes.