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An ex-Marine incarcerated in Iran for nearly two years says in the first letters written to family in Michigan that he is praying for his ailing father and that his grieving mother should not come visit him.

Amir Hekmati's sister Sarah Hekmati told The Associated Press Tuesday that receiving the letters has been "very emotional" for her family, particularly their father, who suffers from brain cancer.

"It's the first time ... we have had any kind of written communication," she said. "The letters speak volumes."

The letters don't offer details about Amir Hekmati's condition, but the 29-year-old wrote that he is "staying strong" and hopes to see them soon in Michigan, according to the Flint Journal.

"I am always praying for you that God cures you," Hekmati wrote to his father, Ali Hekmati, in one of three letters translated from Farsi into English that Sarah Hekmati provided to the AP.

"As for me, do not worry about me," the letter continued. "Overall I am healthy and I am not in need of anything except to see you and to know that you are healthy."

To his mother, Behnaz Hekmati, Amir Hekmati wrote, "For now it is better that you do not come to Iran to see me. I would rather see you again in our own home. ... I have a lot of hope that all of this will be resolved soon."

Hekmati's family says he went to Iran in 2011 to visit his grandmothers. Iran accuses Hekmati of spying, but a previously issued death sentence was overturned. U.S. officials deny the charge.

Hekmati was born in Arizona and grew up in Michigan, where his parents and sister still live, and carries U.S. and Iranian passports.

Recently, his fortunes have improved. He was transferred earlier this year to a less-restrictive environment after 16 months of solitary confinement. Authorities also permitted an uncle in Iran to visit him.

Still, the family has been seeking help from officials in Washington. Sarah and Behnaz Hekmati were among those who traveled to Washington last month to meet with State Department officials and lawmakers, and family members will return next week. State Department officials have said freeing Hekmati is a top priority and diplomatic efforts continue.

Oman, a Gulf state that has served as a mediator between Washington and Tehran before, announced late last month that an Iranian scientist held by the U.S. since 2011 had been released and arrived there. Mojtaba Atarodi was in U.S. custody over allegations he bought advanced technological equipment in violation of U.S. sanctions on Iran.

Sarah Hekmati hopes that Iran sees Atarodi's release as an "olive branch," and spurs efforts to free her brother.

"We're kind of holding our breath," she said. "We realize that the U.S. did that and hope that Iran will look at that and receive that in a positive way."