A federal appeals court has revived two lawsuits challenging the highly secretive National Security Administration's warrantless monitoring of Americans' communications since 9/11.

Two groups of telecom customers had sued the federal government for violating their privacy in the surveillance program authorized by President George W. Bush. Government lawyers have moved to stop such cases, arguing that defending the program in court would jeopardize national security.

In 2009, a San Francisco-based federal judge dismissed the lawsuits. He said the plaintiffs could not sue because they didn't have enough evidence linking the telecom companies to the NSA program.

But the 9th U.S. Circuit Court of Appeals ruled Thursday that the plaintiffs are eligible to sue.

The cases now go back to the district court.