GENEVA (AP) — U.N. human rights experts say Russia violated international law by imprisoning Wall Street Journal reporter Evan Gershkovich and should release him “immediately.”

The Working Group on Arbitrary Detention, made up of independent experts convened by the U.N.’s top human rights body, said there was a “striking lack of any factual or legal substantiation” for spying charges leveled against Gershkovich, 32.

The five-member group said Gershkovich’s U.S. nationality has been a factor in his detention, and as a result the case against him was “discriminatory.”

“The Working Group finds that Mr. Gershkovich’s deprivation of liberty constitutes a violation of international law on the grounds of discrimination based on his nationality,” the group said in a decision that was taken in March but made public only on Tuesday.