Three US citizens accused of espionage after being detained in Iran are set to stand trial more than four months after they entered the country "illegally", Iran's foreign minister has said.

Manouchehr Mottaki told state television on Monday that the three Americans - identified as Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, had entered the country with "suspicious aims".

"Interrogation of the three Americans who have illegally entered Iran with suspicious aims is ongoing," he said.

"They will be tried by Iran's judiciary system and judiciary verdicts regarding their case will be issued."

The two journalists and a photographer were captured in July after apparently crossing the border from northern Iraq.

They are being held in Iran's Evin prison, where Swiss diplomats have visited them and said they are healthy.

In November, Iranian authorities accused the three of being spies, an offense punishable by death under Iranian law. It was not clear, however, if the three had been formally charged with espionage.

Mahmoud Ahmadinejad, Iran's president, has said the hikers "trampled the law" and "need to be punished".

But the US government and the families of the detained say they were on a hiking holiday and crossed the border by mistake.

Hillary Clinton, the US secretary of state, called again on Monday for Iranian authorities to release the three "as soon as possible".

She said the accusations against them are "totally unfounded" and that the trio was in no way involved in any questionable activity.

The incident has further strained ties between Tehran and Washington, which have had no diplomatic relations for nearly three decades.

Iran has also brought up the case of its nationals detained in the US, in response to calls to release the three hikers.

Iranian officials have accused the US of abducting Shahram Amiri, an Iranian nuclear scientist who went missing in Saudi Arabia earlier this year.

The US state department has declined to comment on the Iranian claims.