Reza Saberi, father of freelance reporter Roxana Saberi, said his daughter is "very, very weak and frail" and that he feared for her life.
"She is in bad condition. She can hardly stand up," he told Reuters. "I'm worried about her health. I'm worried about her life."
U.S.-born Roxana Saberi, 32, was sentenced to eight years in jail on April 18. She began refusing food Tuesday to protest her imprisonment.
Her parents visited her in Tehran's Evin jail on Sunday and were unsuccessful in convincing their daughter to stop her hunger strike.
Saberi's case has been the latest source of tension between the U.S. and Iran. Few details have been released on the charges against Saberi, but a spokesman for Iran's Foreign Ministry has implied that Iran has substantial evidence against her, according to a FOX report.
An Iranian investigator alleged that the journalist was passing classified information to U.S. intelligence services.
Meanwhile, the U.S. has issued several demands for Saberi's release.
President Barack Obama, who has expressed grave concern for Saberi's well-being, said at a press conference that he believes she was wrongfully convicted.
"She is an American citizen, and I have complete confidence that she was not engaging in any sort of espionage," he said. "She is an Iranian-American who was interested in the country which her family came from. And it is appropriate for her to be treated as such and to be released."
Iranian President Mahmoud Ahmadinejad has also intervened in support of Saberi, urging that she should be allowed to offer a full defense at her appeal.
Saberi, a former beauty queen from North Dakota who wrote for the BBC and NPR, pledged last week to continue the strike until she is freed.