"We are concerned by recent reports that Kurdish activist Mohammed Seddigh Kaboudvand and human rights defender Narges Mohammadi are suffering from rapidly deteriorating health while in prison," US State Department spokeswoman Victoria Nuland said in a statement.
Kaboudvand "has been on hunger strike since May 26," protesting "Iranian authorities' refusal to allow him to visit his ill son."
Mohammadi "has been denied proper medical care for previous health problems."
"Both (men) are reportedly in critical condition," according to Nuland.
Washington calls "upon Iranian authorities to immediately release all political prisoners and to uphold its own laws and international obligations that guarantee freedom of expression, religion, opinion, and assembly for all citizens."
Iran and the US do not maintain full diplomatic relations, and the US has repeatedly called on the Islamic Republic to observe human rights, particularly in its treatment of political opposition and religious minorities.
Secretary of State Hillary Clinton returned Monday from Israel, where she pledged to "use all elements of American power to prevent Iran obtaining a nuclear weapon."