An international media rights group has expressed concern for the health of a jailed Iranian journalist and human rights activist who suffered a heart attack in a Tehran prison earlier this week.

Paris-based Reporters Without Borders urged Iranian authorities in a statement Friday to release Emadeddin Baghi from prison. The group said Baghi was rushed to a hospital Wednesday after suffering a "double heart attack," and was taken back to the prison late Thursday.

It said Baghi's lawyer told the group that the activist's deteriorating health is due to what he called "appalling" prison conditions and harassment during interrogations.

The United States also expressed concern today for Baghi's welfare as well as the health of jailed Iranian union leader Mahmoud Salehi. A State Department spokesman Tom Casey said the men's health has worsened significantly during their detention.

Casey said Salehi was hospitalized earlier this month after experiencing complications from the lack of proper treatment for chronic kidney disease.

Salehi is a spokesman for Iran's Organizational Committee to Establish Trade Unions. He was arrested in April on charges relating to his trade union activities and was imprisoned in western Iran's Kurdistan region.

Iranian authorities arrested Baghi in October on charges of violating national security.

The State Department says Baghi is being persecuted for drawing attention to the recent increase in executions in Iran and for calling for the humane treatment of prisoners.