Blix warning
The US sanctions, revealed on Thursday, specifically targeted the Islamic Revolutionary Guard Corps (IRGC), which were accused of spreading weapons of mass destruction, and the IRGC's elite Quds Force, which was labelled a supporter of terrorism.
Three Iranian state-owned banks were also blacklisted, along with IRGC-controlled companies and the logistics arm of Iran's defence ministry, as Washington stepped up a drive to freeze Iran out of global banking.
Speaking to Al Jazeera, Hans Blix, the former head of the UN weapons inspection team in Iraq, drew a parallel between the current crisis and the situation in Iraq prior to the US invasion in 2003.
He said: "[The US] hasn't found anything that implicates a will on the part of Iran to have nuclear weapons.
"I think US public opinion is so tired of military adventures that I really doubt that [a military operation] would happen. But the risk is not non-existent. It is there.
"[The] accusations against Iran remind us about Iraq. They say the Iranians are supporting various guerilla groups and militias in Iraq.
"I'd like to see evidence. Accusations don't amount to much."
Internal criticism
Meanwhile, Iran's biggest reformist party has openly challenged Mahmoud Ahmadinejad's, Iran's president, nuclear programme following the sanctions.
The Islamic Iran Participation Front warned of an escalating crisis with the international community, calling for a review of Tehran's nuclear policy.
At a meeting of the party attended by 200 people, Mohsen Mirdamadi, the party's secretary-general, said: "The government should refrain from its adventurous policies."
Mirdamadi criticised Ahmadinejad's anti-Western rhetoric, saying that Tehran had become increasingly isolated since he took office in 2005.
Mirdamadi said: "Are we allowed to impose hardship of [UN] sanctions and other harsh measures on our nation as a result of our illogical and unreal self-glorification?
"Are we allowed to display a brutal and adventurous image of the Iranian nation by our misbehaviour and through making inappropriate speeches?"
Criticising the handling of Iran's nuclear policy is unusual and sensitive because it is considered a matter of national security.
The reformists argue Iran should return to suspending enrichment, the policy under Mohammad Khatami, the former president, who attended the meeting.
Military action
A senior member of the party at the meeting said Ahmadinejad's government was unable to handle the nuclear situation correctly.
Speaking anonymously, he said: "As a result of Ahmadinejad's wrong policies, Iran's military force was tagged as terrorist. It is a step toward military confrontation. Iran should accept temporary suspension to avoid more harm that could not be compensated."
Iranian officials have repeatedly voiced confidence that Washington is not in a position to take military action against Iran, particularly while it is still fighting in Iraq and Afghanistan.
However, Mohammad Ali Abtahi, a cleric and close ally of Khatami, said Ahmadinejad's policies were pushing Iran towards war.
Abtahi said: "It is a very sensitive time... Military action [against Iran] is not far from reality."
Speaking on the sidelines of the meeting to Reuters, Mirdamadi said: "Ahmadinejad has adopted wrong tactics, which is pushing the country towards serious confrontation with the world."