A 46-year-old Iranian-American says a court in northeastern Iran has sentenced him to 18 years in prison after convicting him of 'collaborating with a hostile government.'

Details of the trial and sentencing of Gholamrez Reza Shahini emerged Tuesday, after Shahini made contact with family members in the United States and after he managed to speak by telephone with the Los Angeles Times from a jail cell in the Iranian city of Gorgan.

Shahini said he was convicted Saturday after a three-hour court proceeding.

Shahini, who emigrated to the United States in the early 2000s, was arrested in July while visiting his mother near Gorgan. His family said he was detained by the Islamic Revolutionary Guard Corps while walking to a restaurant with friends.

He told the Times that prosecutors presented evidence gleaned from Facebook posts in 2009 and from other social media in which he voiced support for the pro-democracy 'Green Movement' protests that rocked Iranian cities that year.

A report by the online news service worldtribune.com quoted Iranian human rights activist Hadi Ghaemi, executive director of the International Campaign for Human Rights in Iran, as saying the prison sentence was 'unprecedented' and 'extraordinarily harsh.'

U.S. State Department spokesman John Kirby on Tuesday voiced concern for Shahini's fate, but did not provide immediate details on the situation or disclose diplomatic efforts to gain access to Shahini.

Iran does not recognize dual citizenship rights for anyone born an Iranian citizen. This prevents Western diplomats from visiting detainees or attending their legal proceedings.

Shahini is one of several dual citizens arrested, convicted and sentenced to prison terms in Iran since the Tehran government reached a deal last year to curtail Iran's nuclear program.

Iranian-American businessman Siamak Namazi and his 80-year-old father, Baquer Namazi - himself a former Iranian provincial governor - were sentenced last week to 10-year prison terms for their alleged roles as spies.

Iranian media said four other dual citizens were convicted along with Siamak Namazi and his father.