The White House admitted that the chances of reaching a deal to curb Iran's nuclear program were only 50 percent.
"The likelihood of success in these diplomatic talks are at best 50/50," said White House spokesman Josh Earnest.
The comments come as President Barack Obama's administration comes under sustained pressure from Republican opponents and regional allies in Israel and Saudi Arabia to take a harder line on Iran.
The Republican-controlled US Congress wants to deepen sanctions against Iran, a move that Obama has threatened to veto.
The White House says talks must be given time to succeed, but must not be used by Iran "as cover" to ease mounting diplomatic pressure.
The complex agreement would see Iran subjected to intrusive monitoring and curbs on operations that could lead to the creation of a nuclear weapon.
While negotiators struggle to reach a March target date to reach a framework agreement, there were further signs of the difficult relationship between Washington and Tehran.
The White House on Friday expressed concern about Iran's influence with a Yemeni militia that has besieged the capital Sanaa and forced the resignation of a US-friendly government.
