Iran brought new espionage charges against an Iranian-American scholar who was already convicted of spying and sentenced to 15 years in prison in the country's crackdown following June's disputed presidential election, a human rights group said Thursday.

The new charges raise the possibility of a harsher penalty against Kian Tajbakhsh, a 47-year-old scholar who was in Iran working on a book when he was arrested at his home nearly five months ago amid security forces' postelection sweep against the opposition.

Tajbakhsh was among more than 100 people — most of them opposition activists and protesters — brought before a court in a mass trial criticized by the opposition and rights groups as a show trial.

He was sentenced by a branch of Iran's Revolutionary Court last month to 15 years in prison after being convicted of espionage and endangering state security. It is the harshest prison term handed down so far by the court. His family has denied the charges against Tajbakhsh.

Earlier this week, Tajbakhsh was brought before another branch of the Revolutionary Court that the elite Revolutionary Guard military corps has used to pursue dissidents, and he was charged with additional counts of espionage, the New York-based International Campaign for Human Rights in Iran said in a statement.

The charges had been brought by the Guard, a member of Tajbakhsh's family said, speaking on condition of anonymity because of the sensitivity of the situation. The Guard has spearheaded the crackdown against pro-reform politicians, activists and protesters, accusing them of plotting a Western-backed "velvet revolution" against Iran's clerical-led Islamic Republic.

Tajbakhsh, a social scientist and urban planner, was the only American detained in the crackdown that crushed giant street protests by hundreds of thousands of people after the June 12 election. The opposition claims the vote was rigged in favor of President Mahmoud Ahmadinejad.

U.S. Secretary of State Hillary Rodham Clinton had called for his release.

On Thursday, White House spokesman Robert Gibbs said the United States was deeply concerned about the reports of new charges against Tajbakhsh.

"The charges against Mr. Tajbakhsh are baseless, and his original sentence on Oct. 20 was an outrage," Gibbs said. "The Iranian government cannot earn the respect of the international community when it violates universal rights and continues to imprison innocent people."

Initially, Tajbakhsh's lawyer said he was sentenced to "at least 12 years" in the initial conviction, but it has since been confirmed to be 15 years.

Several high-profile defendants in the mass trial have been released on bail in recent days.

Two more were freed Thursday. One of them, prominent political activist Behzad Nabavi, was sentenced to six years in prison but is suffering from heart disease and has been hospitalized over the past weeks in Tehran.

He is a close ally of Mir Hossein Mousavi, the main opposition candidate who says he was robbed of the presidency through massive fraud in the June election.

The 67-year-old Nabavi was released on $800,000 bail and will undergo further hospital treatment, said his lawyer, Saleh Nikbakht.

In jail since June, Nabavi was one of several former officials brought before the court in the mass trial. He was a deputy speaker of parliament in the 1990s and a deputy prime minister in the 1980s whose negotiations with U.S. officials led to the release of the Americans taken hostage in 1979 during the takeover of the U.S. Embassy.

The other activist released on bail is opposition leader Mousavi's brother-in-law, Shapoor Kazemi. He was freed on $50,000 bail, according to the semi-official Fars news agency.

Kazemi, the brother of Mousavi's wife, was arrested in June as security forces crushed the pro-reform protests that followed the election.

Mousavi's wife, Zahra Rahnavard, said Kazemi, a communications engineer, was apolitical.

The opposition denounced his arrest as an attempt to pressure Mousavi.