By Bozorgmehr Sharafedin and Yeganeh Torbati

DUBAI/WASHINGTON (Reuters) - Iran confirmed on Sunday the detention of an Iranian-American visiting the country, the latest in a string of arrests of dual nationals in the past year.

Asked about reports of the arrest on national security charges, Judiciary spokesman Gholamhossein Mohseni Ejei told a weekly news conference: "The report on the arrest of an Iranian-American dual national is correct," the semi-official Fars news agency said.

Mohseni Ejei declined to name the individual and said he did not know what specific charges would be lodged. He added that the individual was arrested in the northeastern city of Gorgan, but the trial "may be held" in Tehran.

An Iranian-American man, California-based Robin Reza Shahini was detained while visiting his mother in Gorgan earlier in July, according to Shahini's friends.

Asked about Shahini's case during a news conference last week, U.S. Secretary of State John Kerry said he could not say anything about it "at this point in time." A State Department official said last week that the United States had seen the reports of a possible detention of a U.S. citizen, and were looking into it.

In the past nine months, Iran's Revolutionary Guards have arrested at least six dual-national Iranians, their friends and family members say, the highest number of Iranians with dual-nationality detained at one time in recent years to have been acknowledged. The government has confirmed most of the detentions, without giving details of any charges.

The Iranian government does not recognize dual nationality, which prevents relevant Western embassies from seeing individuals who have been detained.

In March, the U.S. State Department issued a warning noting that Iranian-Americans are particularly at risk of being detained or imprisoned if they travel to Iran.

Several Iranian dual nationals from the United States, Britain, Canada and France are being kept behind bars on various charges, including espionage or collaborating with a hostile government. Some are kept to be used for a prisoner exchange with Western countries, according to prisoners, their families and diplomats.

Shahini, in his mid-40s, graduated this spring from San Diego State University, where he studied international security and conflict resolution, said his friend and former classmate, Jasmine Ljungberg. He is set to start a master's program in homeland security at the university in the fall, she said.

Ljungberg was last in touch with Shahini via the Whatsapp messaging app on July 9, when Shahini messaged her to share some pictures of a visit to Iran's mountainous countryside with his family. Ljungberg said she has been in touch with Shahini's girlfriend and briefly with his sister since he was detained.

"Being a student, it was his dream," Ljungberg said. "He has this passion and this drive to change things."

Shahini comes from a poor background, Ljungberg said, and worked a number of jobs after he moved to the United States around 2000 or 2001, including owning a pizza shop and managing a car dealership, before becoming a full-time student.

Shahini was active on social media, including two Facebook accounts, and maintained a blog, but the blog and one of his Facebook accounts appear to have been disabled, Ljungberg said. His Whatsapp account shows someone used the account as recently as Wednesday, July 20, she said.

(Additional reporting by Lesley Wroughton in Washington; Editing by Clelia Oziel)