The US called on Iran Friday to release three Americans hikers who have been held for a year, saying their continued detention is 'unjustifiable'.

'We call on Iran to do the right thing and allow these three Americans to return home to their families,' State Department spokesman P.J. Crowley said.

Saturday marks one year since Iranian authorities arrested Shane Bauer, Sarah Shourd and Josh Fattal, who were hiking in Iraq's Kurdistan region when they allegedly crossed an unmarked border into Iran.

Iranian authorities have suggested they were spying but have not charged them with a crime. Iran has said it is willing to swap them for prisoners in US custody. Their families were allowed to visit them for the first time in May.

'Their release by the Iranian government is long overdue and their continued detention is unjustifiable,' Crowley said.