The United States and Iran clashed Monday on the prospect of a prisoner swap, with a US official mockingly urging the adversary to send a plane to repatriate its citizens.

An Iranian government spokesman said Sunday that Tehran had offered "some time ago" to exchange all Iranian and US prisoners but was waiting for a response from the United States.

He received a reply of sorts Monday via social media as Ken Cuccinelli, the acting deputy homeland security secretary, insisted that the United States had been trying to send back Iranian nationals.

"We have 11 of your citizens who are illegal aliens who we have been trying to return to your country," tweeted Cuccinelli, who is known for his hardline views against immigration.

"You suddenly SAY you want them back, so how about you send a charter plane over and we'll return all 11 at once?" he wrote, attacking the credibility of Iranian Foreign Minister Mohammad Javad Zarif.

Iranian foreign ministry spokesman Abbas Mousavi hit back on Twitter, noting that Zarif has been publicly proposing a prisoner swap since September 2018, and told Cuccinelli: "Stop talking rubbish!"

"Your regime has reacted callously and risked their lives. (The world) is watching your action, not your word. Let our citizens go!"

Cuccinelli charged that the United States has been trying to send back Sirous Asgari, an Iranian scientist acquitted in November on US charges he stole secrets while on an academic visit to Ohio.

The Iranian foreign ministry said that Asgari contracted the COVID-19 coronavirus while in US custody.

Asgari told The Guardian in March that the Immigration and Customs Enforcement agency was keeping him in a detention center in Louisiana without basic sanitation and refusing to let him return to Iran despite his exoneration.

The State Department has been more cautious about Iranian statements that the two nations, both hard hit by the coronavirus, are moving ahead on a prisoner swap.

Secretary of State Mike Pompeo on Monday thanked the efforts of Switzerland, which represents US interest in Iran in the absence of diplomatic relations, in a phone call with Foreign Minister Ignazio Cassis.

State Department spokeswoman Morgan Ortagus said Sunday -- a day before Cuccinelli used Twitter to urge Zarif to send a plane -- that "we do not conduct sensitive diplomacy through the media."

The Islamic republic in December freed Xiyue Wang, a US academic, in an exchange for scientist Massoud Soleimani and said it was open to further swaps.

Iran is holding US citizens Siamak Namazi, convicted on charges that include espionage, his father Baquer and environmental expert Morad Tahbaz.

Relations have sharply deteriorated since two years ago when President Donald Trump withdrew from a nuclear accord and unilaterally imposed sweeping sanctions.