Amir Hekmati maintains his innocence and repeats in the letter that he is wrongly imprisoned.

The family of a former U.S. Marine imprisoned in Iran says he is going on a hunger strike and has dictated a letter asking President Obama not to forget him as dialogue continues between the two countries over Iran’s nuclear activities.

Amir Hekmati told the family by phone Tuesday about the hunger strike, Sarah Hekmati said in an email to The Associated Press.

“We are truly devastated by this news but know that he is struggling and losing hope for a resolution to his case,” his sister said.

Amir Hekmati holds dual U.S. and Iranian citizenship. He was born in Arizona and raised in Michigan.

In August 2011, Hekmati was arrested in Iran, then tried, convicted and sentenced to death for spying. Hekmati appealed, and Iran’s Supreme Court annulled the death sentence and ordered a retrial in 2012.

The country’s Revolutionary Court then overturned his conviction for espionage, instead charging him with “cooperating with hostile governments” and sentenced him to 10 years in prison.

The U.S. government repeatedly has denied that Hekmati is a spy. His family, which lives in Flint, Michigan, says he is innocent and only went to Iran to visit his grandmothers.

Sarah Hekmati said her brother dictated the letter Monday and asked that it be addressed to Obama. She emailed a copy of the letter Tuesday night to the AP.

“It is my hope that after reading this letter you, or anyone who may see this, will help end the nightmare I have been living,” the letter reads.

“Every day, I wake hoping that there is news of my release. Every night, I go to sleep disappointed to mark another day that I am still behind these prison walls, away from my family, friends and meaningful human contact. Away from my father who is gravely ill. There is no end in sight.”

His father, Ali Hekmati, has been diagnosed with brain cancer.

Amir Hekmati maintains his innocence and repeats in the letter that he is wrongly imprisoned, but is concerned that his future is now tied to ongoing negotiations between the U.S. and Iran over whether to put long-term curbs on Tehran’s nuclear program in exchange for sanctions relief.

“I ask that you not forget me, Mr. President,” Hekmati says in the letter. “I ask that you make it clear that my case is unrelated and should be resolved independent of your talks. I ask that your team impress upon the Iranian officials that more than three years without resolution is simply too long.

“I know that the climate between the United States and Iran is delicate. But I should not fall victim to it.”

Hekmati is being held in Iran’s notorious Evin prison, north of Tehran.

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