An Iranian lawyer says a physics student arrested in Tehran earlier this year, while on leave from an American university, has gone on trial on suspicion of having relations with a hostile country.

The student's lawyer, Saeed Khalil, says authorities also accused the man of receiving illicit payments. Khalil says the student, Omid Kokabee, denied both charges at the trial's opening session on Tuesday.

Khalil says Iranian authorities barred him from contact with Kokabee ahead of the hearing.

Kokabee was arrested at Tehran airport in February, as he prepared to get on a plane to the United States to resume his studies after a vacation in Iran. Kokabee, who is in his late 20s, is a graduate student at the physics department of the University of Texas.

No other details about the accusations against the student were immediately available. It is not clear when the trial will resume.

Tehran and Washington have not had diplomatic relations since cutting them off after Iran's 1979 Islamic Revolution. Several U.S.-Iranian dual nationals and Americans of Iranian descent have faced arrest and criminal charges while visiting Iran in recent years.