The three Americans were arrested July 31 after straying over the Iranian border from northern Iraq. The U.S. government and their families say there were on a hiking vacation and crossed accidentally.
Mottaki said at a news conference Monday that the Americans had "suspicious aims."
Tehran chief prosecutor Abbas Jafari Dowlatabadi said last month that the three "have been accused of espionage" and that investigations were continuing, according to the state news agency IRNA. He said an "opinion (on their case) will be given in the not distant future."
In Iran's opaque judicial system, the process of indictment and trial often takes place behind closed doors.
When the accusation of espionage was leveled in November, U.S. Secretary of State Hillary Rodham Clinton repeated her call on Iran to release Shane Bauer, Sarah Shourd and Josh Fattal.
Clinton said, "We believe strongly that there is no evidence to support any charge whatsoever.
"And we would renew our request on behalf of these three young people and their families that the Iranian government exercise compassion and release them, so they can return home."
The announcement of a pending trial comes as the U.S. and its allies that Tehran will be subjected to tougher sanctions over its refusal to halt uranium enrichment, and raises the possibility that the country's leaders are using the case to pressure the U.S. in the nuclear negotiations.