It's time for the United States to hold Iran accountable.

The families of four prisoners detained in Iran say Tehran is using them to extract political and financial concessions from Washington. Those families will testify before Congress Tuesday.

President Trump issued a sharp warning of "serious consequences" unless Iran releases all prisoners and stops using "hostage taking as a tool of state policy."

CBS News' Margaret Brennan spoke to one of the families.

"Everything I hear terrifies me, and it terrifies me more and more every day," said Babak Namazi. Namazi is pleading for the release of his brother 45-year-old Siamak Namazi and 80-year-old father Baquer - imprisoned in Iran for nearly two years.

"All I ask for obviously is for their safe return and for their immediate return before it's too late," Namazi said.

Asked what he means by, "before it's too late," Namazi said, "Well what I mean is the conditions of both my father and Siamak are horrible. My father is not doing well."

White House officials told CBS News they were increasingly concerned about Baquer's deteriorating health and say Siamak has been mistreated due in part to his U.S. citizenship.

Baquer and Siamak are being held inside Iran's notorious Evin Prison - something Iranian Foreign Minister Mohammad Javad Zarif denied in an earlier interview with Brennan.

Click here to read more.