Human Rights Watch said Wednesday it filed a suit alleging the US Drug Enforcement Administration illegally collected records of its phone calls to foreign countries for years.
The lawsuit comes after a series of media reports and disclosures in public documents revealing a surveillance program dating back to the 1990s, which reportedly collected data on virtually all international phone calls.
"At Human Rights Watch we work with people who are sometimes in life-or-death situations, where speaking out can make them a target," said Dinah PoKempner, general counsel at the watchdog group.
"Whom we communicate with and when is often extraordinarily sensitive -- and it's information that we wouldn't turn over to the government lightly."
The suit filed in federal court in California asks a judge to declare the surveillance a violation of the group's constitutional rights, and to purge all records from the program.
"The DEA's program is yet another example of federal agencies overreaching their surveillance authority in secret," said Mark Rumold, staff attorney at the Electronic Frontier Foundation, which is representing the human rights organization.
"We want a court to force the DEA to destroy the records it illegally collected and to declare -- once and for all -- that bulk collection of Americans' records is unconstitutional."
According to the complaint, the DEA disclosed the existence of its mass surveillance program in January 2015, after a federal judge ordered the government to provide more information in a criminal case against a man accused of violating export restrictions on goods to Iran.
News reports have said that the operation gathered records on calls to over 100 countries.
USA Today reported this week that the DEA program -- aimed at tracking drug traffickers -- harvested data from more than one billion calls from as early as 1992.