WASHINGTON - The families of three US hikers detained in Iran voiced concern Sunday about their loved ones' mental state after weeks of semi-isolation in a Tehran prison, and are asking people to send them messages of support.

"Sarah, Shane and Josh are being well treated and seem to be physically well, but we are more and more worried about their state of mind," Nora Shourd, mother of one of the detainees said in a statement.

Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27 were arrested July 31 after straying - inadvertently according to their families - into Iranian territory during a hiking trip in Iraq's Kurdistan region.

Held at Tehran's Evin prison in separate cells and with few visits allowed, the trio faces espionage charges, despite denials from friends and relatives that they are spies and US insistence that they be released.

"Our children are virtually cut off from the world outside their jail cells and have hardly any contact with each other," Nora Shourd said.

Shourd and the other families have set up a special post office box where messages can be sent to the three detainees at Evin prison.

The messages, they said, are meant to ease the isolation of their three loved ones.

"It's vital for them to know they're getting support and one way to do that is to send them messages. It also shows the Iranian authorities that people care and won't let them be forgotten," said Laura Fattal, mother of Josh Fattal.

"We know from the consular visits that our kids are aware of how hard people are working to secure their release and they have received some letters," she added.

Monday Tehran's chief prosecutor Abbas Jaffari Doulatabadi said investigations were continuing against the trio, the official IRNA news agency reported. "The three Americans arrested near the border of Iran and Iraq are facing charges of spying and the inquiry is continuing," he said.

But Tuesday Iran's top diplomat seemed to sidestep the spying charges. "The crime they committed is of illegally entering Iranian territory. The other things are at the level of accusations," Foreign Minister Manouchehr Mottaki told reporters, indicating that the trio may not have been formally charged with spying as reported on Monday.

The families said Swiss diplomats have visited the three Americans "only twice" since they were locked up. On the last visit in October, one diplomat said the trio were in good physical shape.

Switzerland has looked after US interests in Iran since Washington and Tehran broke off diplomatic relations following the seizure of the US embassy by radical students in the aftermath of the Islamic revolution of 1979.