WASHINGTON (Reuters) - The United States is troubled by reports that an Iranian-American man may have been sentenced by Iran to 18 years in prison, a State Department spokesman said on Tuesday, a week after an Iranian-American businessman and his father were also jailed by an Iranian court.

"We are troubled by reports that Robin Reza Shahini, a person reported to be a U.S. citizen, may have been convicted and sentenced to 18 years in prison," spokesman John Kirby told a daily briefing. He called on Tehran to halt "arbitrary and politically motivated detentions."

California-based Shahini was detained while visiting his mother in Gorgan in July, according to Shahini's friends.

Last week, businessman Siamak Namazi and his 80-year-old father Baquer Namazi, a former Iranian provincial governor and former UNICEF official, were sentenced to 10 years in prison on charges of cooperating with the United States.

Iran does not recognize dual nationality, which prevents Western embassies from visiting such detainees.

Washington secured the release of five U.S. citizens from Iran in January in a prisoner exchange and has said it raises the detention of other Americans held by Tehran during meetings with Iranian officials.

Major world powers, including the United States, reached a landmark nuclear deal with Iran in July last year, which lifted some international sanctions against Iran.

"Nobody is turning a blind eye here, but what is behind this, I don't think any of us know with certainty," Kirby said, when asked why Iran keeps detaining dual citizens.

"I can't speak to the motivation behind these detentions but if one of the motivations was to secure ransom, it is a false perception. It is wrong."

Republican leaders of the U.S. House of Representatives plan a vote as soon as mid-November on a 10-year reauthorization of the Iran Sanctions Act, congressional aides told Reuters.

The move could increase frustrations in Iran, which has been complaining for months that remaining U.S. sanctions have frightened away trade partners.

(Reporting by Lesley Wroughton; Editing by Dan Grebler)