MOSCOW — An ex-U.S. Marine held in Russia on suspicion of spying on Friday accused a security service investigator of subjecting him to threats and asked a court to have the man removed from his case, the TASS news agency reported.

Paul Whelan, who holds U.S., British, Canadian and Irish passports, was detained in a Moscow hotel room on Dec. 28 and accused of espionage, a charge he denies. If found guilty, he faces up to 20 years in jail.

"I call for FSB (security service) investigator captain Alexei Khizhnyak to be called off (the case). He is insulting my dignity and threatening my life," Whelan was quoted as telling the Moscow court.

His comments appear likely to aggravate U.S.-Russia relations already strained over a range of issues, including the war in Syria, economic sanctions and the detention of another U.S. citizen, prominent investor Michael Calvey, who is under house arrest in Moscow on embezzlement charges.

Whelan was appearing at a hearing convened to decide whether to lengthen his his detention. The court ruled on Tuesday to extend his detention by three months.

“Extend the accused Whelan a preventive measure in the form of detention for three months, until August 29, 2019," a judge said.

The judge told Whelan he only had the right under Russian law to request the replacement of court or prosecuting officials, but not an investigator.

"I understand that, but what am I supposed to do if my rights are being violated," Whelan was quoted as saying.

Michael Yoder, senior counselor of the U.S. Embassy Moscow, said he was "disappointed" by the court's ruling.

"Five months have passed since the day of his arrest. There is still no evidence of his unlawful activities. The question arises why Paul Whelan is still in the detention center if there is no evidence," he said. "It would be human to let him go home.”

The U.S. embassy's press secretary, Andrea Kalan, said, "We urge Russian authorities to allow our consular officers to immediately meet with Paul Whelan so that we can directly talk to him about his safety and condition."

The detainee's brother, David Whelan, said on MSNBC that Paul was in Russia for a friend's wedding but never made it to the ceremony. David said his family learned his brother had been detained from online news reports.

In a series of tweets on Friday, David Whelan criticized the court's decision to keep his brother detained.

"#PaulWhelan will be wrongfully detained for another 3 months. Just more Russian legal theater courtesy of the FSB," one tweet read. "These hearings aren't about justice or facts."

Another tweet read, in part: "#PaulWhelan's continued detention is about coercion. The FSB has isolated him to pressure a confession."

Alan Kaykutov reported from Moscow, and Minyvonne Burke from New York