The United States' new ambassador to Russia urged Moscow on Thursday to release a former U.S. Marine accused of spying, and said Russian investigators had failed to present credible evidence to back up their case.

Days after starting as U.S. ambassador, John Sullivan accused Russian authorities of "shameful treatment" of Paul Whelan, who was detained by security agents in a Moscow hotel room on Dec. 28, 2018 and accused of espionage.

Whelan, 49, denies the charges against him and holds U.S., British, Canadian and Irish passports. At court hearings over the past year, he has said he is being ill treated.

The case, in which Whelan will be jailed for 20 years if he is found guilty, has strained U.S.-Russian ties that are already under pressure from an array of issues including the conflicts in Ukraine and Syria and election-meddling allegations.