Iran ruled out any US preconditions for reviving a 2015 nuclear deal on Monday, including the release of American prisoners detained by the Islamic Republic.

"Iran has never agreed to any preconditions set by the US. The comments made by a US official on the release of US prisoners in Iran are for domestic use" Saeed Khatibzadeh, a spokesman for the Iranian Foreign Ministry, said.

On Sunday, the lead US nuclear negotiator said the US was unlikely to reach an agreement with Iran to revive the nuclear pact unless Tehran releases four American citizens that Washington says it is holding captive.

Will 2022 Bring A Revived Iran Nuclear Deal -- Or A Hard-Line Plan B?

The official, US Special Envoy for Iran Robert Malley, reiterated the US' long-held position that the issue of the four people detained in Iran is unrelated to the nuclear talks. However, he moved a step closer to saying that their release was a precondition for a nuclear agreement.

"They're two different things, and we're pursuing both of them. But I will say that getting back into the nuclear deal when four innocent Americans are held hostage by Iran is quite difficult for us" Malley said.

Setting preconditions such as the release of US prisoners held in Iran would slow down indirect negotiations between Tehran and Washington on reviving a 2015 nuclear deal, according to Iran's state news agency IRNA.

"Washington's irrelevant preconditions will make striking an agreement in Vienna difficult," IRNA stated.

Iran is now in talks with world powers in Vienna to revive the 2015 deal, which the US unilaterally left during Trump's presidency in 2018.