Amir Hekmati, a former US marine imprisoned in Iran, has started a hunger strike, his family said, after sending a letter to US President Barack Obama asking not to forget him.

Hekmati told his family about the hunger strike on Tuesday after dictating his letter over the phone to his sister, Sarah Hekmati, The Associated Press reported.

“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.

Hekmati, born in Arizona and raised in Michigan, holds a dual US and Iranian citizenship. However, Iran does not recognise dual citizenship.

Hekmati has been sentenced to 10 years in prison, after the Iranian Supreme Court annulled his death sentence and ordered a retrial in 2012.

His conviction was then overturned for espionage by the Revolutionary Court, charging him with "cooperating with hostile governments" instead.

Letter to Obama

The US government has repeatedly denied that Hekmati is a spy, and his family said his visit to Iran was merely to see his grandmother.

“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,'' Hekmati's letter to Obama 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.”

Hekmati repeats in the letter that he is wrongly imprisoned, but is concerned that his future is now tied to ongoing negotiations between the US and Iran over whether to put long-term curbs on Tehran's nuclear programme in exchange for sanctions relief.

"I ask that you not forget me, Mr. President," he 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 Evin prison, north of Tehran.

Source: AP