The United States has called on Iran to allow an elderly American detained in Iran for more than five years to travel to receive urgent medical care.

Former UNICEF representative Baquer Namazi, 84, requires surgery within days to clear up a severe blockage in the main artery that supplies blood to his brain, said his son Babak.

“We call on Iran to allow him to travel and recover from surgery with his family by his side, “U.S. envoy for Iran Robert Malley said on Twitter.

Earlier, Namazi’s son appealed for his father's immediate release in order to allow him to receive emergency and potentially life-saving surgery.

He was detained in 2016 when he traveled to Tehran to try to win the release of his other son, Siamak Namazi, a businessman arrested in Iran months earlier.

"My father has already lost so much precious time. I'm begging Iran to let him spend whatever small amount of time he has left with his family," Babak Namazi told reporters on October 4.

Both Baquer and Siamak Namazi were sentenced to 10 years in prison in Iran on what the United States and the UN say were trumped-up spying charges.

An Iranian court early last year commuted Baquer Namazi's sentence but his lawyers said authorities have refused to issue him an Iranian passport, which he needs to leave as Tehran does not recognize dual nationality. Siamak Namazi is still serving the sentence in Iran's notorious Evin prison.

In a letter to the UN special rapporteur on the right to health, lawyers for the family said Baquer Namazi's case was "dire and extremely urgent."

Jared Genser, a lawyer for the Namazis, said the family had appealed directly to members of President Joe Biden's administration to press Iran to let Namazi leave.

"The time for action is now. I call on President Biden to engage personally to make this happen," Genser said.

State Department spokesman Ned Price later told reporters the release of the Namazis and other Americans from Iran was a "top priority" for the Biden administration.

With reporting by AFP and AP