The Obama administration has rejected a proposal from Iran to cooperate in the fight against Islamic State (IS) militants in exchange for nuclear programme concessions.
White House spokesman Josh Earnest told reporters on Monday that nuclear talks with Tehran were "entirely separate" from Barack Obama's attempts to build a coalition to destroy IS, also known as ISIS or ISIL.
"The United States will not be in the position of trading aspects of Iran's nuclear programme to secure commitments to take on ISIL," Mr Earnest said.
Earlier, senior Iranian officials told Reuters that Iran was seeking flexibility on its uranium enrichment programme in exchange for Tehran's cooperation in fighting IS.
The United States has led effort to combat the IS militants, who have seized swathes of Iraq and Syria and proclaimed a caliphate.
The extremist group has made rapid territorial gains across the region and released graphic videos depicting the beheading of two US journalists and a British aid worker.
US Secretary of State John Kerry made several stops in the region last week in a bid to help boost cooperation from other nations for the ongoing fight.
The US and France have already launched airstrikes against IS targets, and the UK has not ruled out joining the bombing campaign.