U.S. Secretary of State John Kerry heads to Geneva Friday for talks on Iran's nuclear program, amid signs a deal is within reach.
The State Department says it hopes Kerry's presence will "help narrow differences" in the negotiations between Iran and six world powers.
Both sides reported progress Thursday on a deal requiring Iran to scale back its nuclear program in exchange for sanctions relief.
Iranian Foreign Minister Mohammed Javad Zarif told CNN he believes an agreement can be reached before the close of negotiations Friday.
Iran is seeking an end to sanctions that have devastated its economy, while the six nations want assurances Iran is not building nuclear bombs.
In Washington, President Barack Obama said the deal being discussed would offer "modest relief" from the sanctions, but that most would stay in place.
"We can provide them some very modest, relief, but keeping the sanctions architecture in place, keeping the core sanctions in place, so that if it turned out during the course of the six months when we're trying to resolve some these bigger issues that they're backing out of deal or they're not following through on it, or they're not willing to go forward and finish the job of giving us assurances that they're not developing a nuclear weapon, we can crank that dial back up."
President Obama told NBC News there is a possibility of a phased agreement, the first part of which would stop Iran from further expanding its nuclear program.
It is unclear what Iran is willing to concede. Foreign Minister Zarif said Tehran is not willing to suspend its uranium enrichment program entirely, but would consider scaling it back.
This is the second meeting of the so-called P5+1 talks since Iranian President Hasan Rouhani took power in August, on promises of reaching a nuclear deal with the West.
Observers say Kerry's last minute-decision to head to the talks suggest a deal could be imminent. But negotiators warn the two sides still disagree over how much Tehran will scale back its nuclear program.
Kerry will fly to Geneva later Friday after meeting in Israel with Prime Minister Benjamin Netanyahu, who has warned that a deal that does not include the complete dismantling of Iran's nuclear program would be a mistake.
The U.S. Congress has also taken a harder stance on Iran than the Obama administration. On Thursday, the chairman of the U.S. Senate Banking Committee said he will move ahead with a package of tough new sanctions on Iran after the Geneva session ends.