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

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.

'I call on the Iranian government to immediately release Sarah, Shane and Josh,' Obama said in a statement. 'Their unjust detention has nothing to do with the issues that continue to divide the United States and the international community from the Iranian government.'

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.

'I want to be perfectly clear: Sarah, Shane and Josh have never worked for the United States government,' Obama said. 'They have never had any quarrel with the government of Iran and have great respect for the Iranian people.'

The three hikers' families were allowed to visit them for the first time in May. Obama said Iran's gesture was welcome and that he spoke with the three mothers earlier this week.

But he added: 'I cannot imagine how painful it was for these three courageous women to return home without their children.'