Iranian Foreign Minister Manouchehr Mottaki has said three young Americans suspected of spying entered its territory illegally.
Mr Mottaki told reporters in Tehran that other accusations against the trio were still being investigated.
The US has called for the swift release of Shane Bauer, Sarah Shourd, and Josh Fattal, who were held after allegedly crossing into Iran from Iraq in July.
Their families say they were hiking and strayed across the border accidentally.
Speaking at a news conference in the Iranian capital on Tuesday, Mr Mottaki said a number of accusations against the Americans were being "legally pursued and investigated" by the country's judiciary.
"Illegally crossing over the western borders into Iran is the most basic offence that has been committed," he said.
His comments came hours after President Mahmoud Ahmadinejad said Iran had a right to punish people who broke its laws.
"In all countries, crossing borders would have a very heavy sentence, according to the law," he told reporters in Turkey.
"Hopefully, they will have an appropriate answer in the court, and hopefully they will convince the judge that they did not have any intention of crossing the border illegally."
Mr Ahmadinejad also accused the US of jailing innocent Iranians and mentioned the suspicious disappearances of a nuclear scientist and a top defence official in recent years. The US has refused to comment on their cases, and there has been speculation that they defected.
"Recently an Iranian went to Mecca for pilgrimage and he was kidnapped by the American forces," he said.
"And two years ago, an Iranian was also kidnapped by Americans in Turkey and their families are very much concerned about the fate of these people."
Earlier on Monday, Iran's prosecutor general, Abbas Jafari Dolatabadi, said the three American hikers were "facing charges of spying", according to the state news agency, Irna.
He said an opinion would be given "in the not too distant future".
In a statement, the Americans' families described the spying allegations as "entirely at odds with the people Shane, Sarah and Josh are and with anything that Iran can have learned about them since they were detained".
Meanwhile, US Secretary of State Hillary Clinton said there was "no evidence to support any charge whatsoever".
Swiss diplomats were allowed to meet the trio, who are in their 20s and 30s, in late September for the first time since their arrest.
The Swiss government represents US interests in Iran, with whom the US has no formal diplomatic relations.