Siamak Namazi, a US citizen jailed in Tehran, announced a seven-day hunger strike, starting on Monday, as he implored US President Joe Biden to put greater priority on his case.
"All I want, sir, is one minute of your days' time for the next seven days devoted to thinking about the tribulations of the US hostages in Iran," Mr Namazi wrote in an open letter to Mr Biden.
"Only the president of the United States has the power to bring us home, should he set his mind to do so."
Mr Namazi, 41, who is of Iranian descent, was arrested in October 2015 on charges, he denies, of trying to overthrow the clerical state.
Iran also imprisoned his father, Baquer Namazi, 86, after he flew there in 2016 to try to secure his son's release. He was allowed to leave Iran in October on medical grounds.
Siamak Namazi was granted furlough at the time but then returned to Tehran's infamous Evin prison, which since has witnessed a deadly fire.
He launched his hunger strike on the anniversary of Iran's release of five imprisoned Americans in 2016, the year after Tehran entered a nuclear deal with the US and other world powers.
Former president Donald Trump, who was elected later that year, pulled the US out of the deal in 2018, but his administration managed to obtain the release of two more Americans through prisoner swaps with Iran.
"My captors enjoy taunting me about that fact by saying things like, 'How can your beloved America be so heartless? Not one but two US presidents freed others but left you behind!'" Mr Namazi wrote in his letter.
The Biden administration sought to restore the nuclear deal and made it clear it could not go forward without the release of imprisoned Americans.
But Mr Biden has acknowledged that the deal was effectively dead after protracted negotiations and as attention turned to Iran's deadly crackdown on protests triggered by the death of a woman arrested by the morality police.
"Today the whole world is witnessing how atrociously this regime can respond to those who dare demand their basic rights," Mr Namazi said.
Iran's government generally refuses to acknowledge dual citizenship and on Saturday announced the execution of a British-Iranian citizen, Ali Reza Akbari, convicted of spying for his adopted country.