The Pentagon's top general said on Thursday that he was opposed to removing the elite Quds Force of Iran's Revolutionary Guards from the US terror group list, one of Tehran's conditions for restoring the 2015 Iran nuclear agreement.
Chairman of the Joint Chiefs of Staff Gen Mark Milley voiced his opposition to delisting the group in a hearing in front of the Senate Armed Services Committee.
“In my personal opinion, I believe the IRGC Quds Force to be a terrorist organisation and I do not support them being delisted from the foreign terrorist organisation" list, Gen Milley said.
Iran has pressed for the removal of the State Department's official "Foreign Terrorist Organisation" designation of the Islamic Revolutionary Guard Corps as a condition for returning to the 2015 deal that sought to control its nuclear development and prevent it from acquiring a nuclear weapons capability.
The previous US administration of Donald Trump unilaterally abrogated the deal in 2018 and a year later slapped the FTO label on the IRGC, including its Quds Force.
President Joe Biden has sought in negotiations to salvage the Joint Comprehensive Plan of Action, or JCPOA, as the 2015 deal is officially called.
But talks have stalled in the past month over a handful of final issues, including Iran's demand over the terror designation.
In a statement on the original designation, the State Department said: "The IRGC -- most prominently through its Quds Force -- has the greatest role among Iran's actors in directing and carrying out a global terrorist campaign."
It singled out specific acts by the Quds Force, including attempted bombings and assassinations inside the United States and other countries.
Privately, US officials have said removing the terror designation would not have much concrete impact, because the IRGC remains under a long list of economic and political sanctions.
Both Gen Milley and Secretary of Defence Lloyd Austin warned of Iran’s increasing threats to the US, to regional stability, and the global flow of energy.
“Iran is likely to remain a significant regional threat to the United States and our partners and allies," Gen Milley said.
He argued that in its support for terrorism and "a proxy army inside the borders of its neighbours, along with its ballistic missile programmes, Iran seeks to revise the Middle East regional order and balance of power in Iran’s favour".
He warned that its nuclear programme is a tool it uses to gain leverage towards that goal.
However, Gen Milley said he supports a nuclear deal with Iran to avoid further destabilisation.
“If not constrained through a new diplomatic agreement, their continued nuclear programme threatens the emergence of a regional arms race,” he said.
The top commander accused Iran of openly threatening to assassinate current and former members of the US government and military.
Meanwhile, Mr Austin called for vigilance against Iran's nuclear ambitions.
“Iran challenges Middle East stability and poses threats to US forces, our partners, and the free flow of energy,” Mr Austin said.
The Biden administration is coming under increasing pressure from Congress to avoid making controversial concessions to Iran in any deal.
On Wednesday, 18 House Democrats issued a statement voicing concerns about the negotiations.
One of the signatories is Congresswoman Val Demings, who was on Mr Biden's shortlist of vice presidential contenders in 2020.
“As a 27-year law enforcement officer and chief of police, I know a habitual offender when I see one," she said.
"Iran’s corrupt leaders are habitual offenders, sponsors of terror and a threat to US national security, Israel and stability around the world,” Ms Demings said.
AFP contributed to this report.