"The United States is concerned by reports that Iranian authorities have stepped up their repression of student activists in the wake of the ninth anniversary of the July 9, 1999 student uprising," it said in a statement.
Mohammad Hashemi and Bahareh Hedayat were arrested and sent to the notorious Tehran prison of Evin on Sunday, the Etemad newspaper said in Tehran. The two are members of the central council of the pro-reform student group The Office to Consolidate Unity.
Their detention raised the number of students arrested throughout the country to around 18 since the ninth anniversary of the student protests, the Kargozaran newspaper said.
Groups of students hold small gatherings annually to commemorate July 9, which marks the most significant clash between the authorities and students in the Islamic republic.
The riots were sparked when hardline vigilantes stormed student dormitories and the ensuing violence left dozens of students wounded and many arrested.
"The United States stands with all students and civil society leaders in Iran who courageously struggle for universal rights and justice in their country," said the statement from State Department spokesman Sean McCormack.
"We continue to work with the international community through the United Nations and foreign governments to focus attention on the Iranian regime's human rights abuses," it added.
"The United States calls on the Iranian regime to guarantee fundamental freedoms to all its citizens, including the provision of due process and fair trials."
