Tension between the US and Iran rose sharply today after Tehran said it would put on trial three Americans who apparently entered the country illegally during a hiking trip.

Hillary Clinton, the US secretary of state, described accusations against the three as "totally unfounded" and called for their immediate release. The three had been on a hiking trip in the mountains on the border between Iran and Iraqi Kurdistan in July when they were arrested by the Iranians.

A high-profile trial in Iran of the Americans would be an emotive issue in the US and would threaten to become a cause célèbre at a time when relations between Washington and Tehran are already strained over the nuclear stand-off.

The Iranian foreign minister, Manouchehr Mottaki, said the three had "entered Iran illegally with suspicious aims". An Iranian prosecutor last month accused them of spying.

Mottaki said: "They will be tried by the Iranian judiciary system and verdicts will be issued." He did not say what they had been charged with.

The decision contrasts with Iran's decision earlier this month to release British yachtsmen who had been held by Iranian revolutionary guards for a week after straying into the country's waters. Iran initially threatened to charge the crew.

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