The US Court of Appeals for the Ninth Circuit said on Wednesday the warrantless telephone dragnet that secretly collected millions of records violated the Foreign Intelligence Surveillance Act and may well have been unconstitutional.

Snowden, who fled to Russia in the aftermath of the 2013 disclosures and still faces US espionage charges, said on Twitter the ruling was a vindication of his decision to go public with evidence of the National Security Agency's eavesdropping.

"I never imagined that I would live to see our courts condemn the NSA's activities as unlawful and in the same ruling credit me for exposing them," Snowden said.

Evidence the NSA was secretly building a vast database of the who, how, when and where of millions of mobile calls was the first and arguably most explosive of the Snowden revelations published by the Guardian newspaper in 2013.

Until that moment, top intelligence officials publicly insisted the NSA never knowingly collected information on Americans at all.

After the program's exposure, US officials fell back on the argument the spying was crucial in fighting extremism, citing the case of four San Diego residents accused of aiding religious fanatics in Somalia.

They insisted that the four - Basaaly Saeed Moalin, Ahmed Nasir Taalil Mohamud, Mohamed Mohamud and Issa Doreh - were convicted in 2013 thanks to NSA program.

However the Ninth Circuit ruled Wednesday the claims were "inconsistent with the contents of the classified record".

The ruling will not affect the convictions of Moalin and his fellow defendants; the court ruled the illegal surveillance did not taint the evidence introduced at their trial.

Nevertheless, watchdog groups including the American Civil Liberties Union, which helped bring the case to appeal, welcomed the judges' verdict.

"Today's ruling is a victory for our privacy rights," the ACLU said in a statement, saying it "makes plain that the NSA's bulk collection of Americans' phone records violated the Constitution".

Australian Associated Press