Roxana Saberi, a 31-year-old dual American-Iranian citizen, was convicted of spying for the United States and sentenced to eight years in prison after a swift, closed door trial earlier this month.
Saberi was arrested in late January and initially accused of working without press credentials, but earlier this month, an Iranian judge leveled the far more serious allegation of espionage.
"She went on a hunger strike Tuesday to protest her imprisonment. Today is the fifth day," Reza Saberi told The Associated Press. "She will remain on hunger strike until she is freed."
Her father said Roxana's lawyer, Abdolsamad Khorramshahi, has appealed the sentence.
"The lawyer filed his appeal today," he said.
Iran's judiciary chief, Ayatollah Mahmoud Hashemi Shahroudi, ordered a full investigation into the case Monday, a day after President Mahmoud Ahmadinejad urged Tehran's chief prosecutor to ensure Saberi be allowed a full defense during her appeal.
It is the first time Iran has found an American journalist guilty of spying, and it is unclear how it would affect recent overtures by the Obama administration for better relations and engagement with Washington's longtime adversary.
The case has been a source of tension with the U.S. at a time when President Barack Obama is trying to open a dialogue with Iran to end a decades-long diplomatic standoff. The U.S. has called the accusations against Saberi baseless and demanded her release.
Iran's judiciary spokesman Ali Reza Jamshidi said Tuesday that the appeals court would reconsider the verdict, an indication her sentence could be commuted.
Saberi, who was born in the United States and grew up in Fargo, North Dakota, moved to Iran six years ago and worked as a freelance journalist for news organizations including National Public Radio and the British Broadcasting Corp. She received Iranian citizenship because her father was born in Iran.
Iran has released few details about the charges against Saberi but Iran's Intelligence Minister Gholam Hossein Mohseni Ejehi said Tuesday that the initial investigation of Saberi was done by an expert on security and counterespionage at the Intelligence Ministry before her case was referred to the court.
An Iranian investigative judge involved in the case alleged that Saberi was passing classified information to U.S. intelligence services.
Her parents have traveled to Iran from their home in Fargo in a bid to help win their daughter's release.
Saberi's father has said his daughter, who was Miss North Dakota in 1997, had been working on a book about the culture and people of Iran, and hoped to finish it and return to the United States this year.