WASHINGTON (AP) — A U.S. senator has written the Russian ambassador urging Moscow to turn over former National Security Agency contractor Edward Snowden.

Republican Sen. Lindsey Graham of South Carolina said in a letter on Monday that if the U.S. and Russia are to have a constructive relationship, Russia must cooperate with American authorities and apprehend Snowden.

Graham said the case is a critical test of the reset in relations between the two countries.

Graham wrote to Sergey Kislyak, the Russian ambassador to the United States.

Snowden is facing espionage charges stemming from his disclosure of U.S. surveillance programs. He left Hong Kong on Sunday and traveled to Moscow though his whereabouts are unknown.