MOSCOW (Reuters) - Russia accused the United States of trying to break the spirit in jail of Maria Butina, a woman U.S. authorities say is a Russian government agent, and complained she was being subjected to “borderline torture.”

Butina, a 29-year-old gun rights advocate living in Washington, D.C., was arrested in July and has been accused of acting as a Russian government agent while developing ties with American citizens and infiltrating political groups.

Butina, who pleaded not guilty, was detained without bail pending trial after prosecutors presented evidence suggesting she had connections to Russian intelligence operatives and was being funded by Russian oligarchs.

Her case, which Russian authorities allege is fabricated and politically-motivated, has become the latest in a series of irritants between Washington and Moscow at a time when ties are already languishing at a post-Cold War low over allegations of Russian interference in U.S. politics.

Russia’s U.S. embassy said in a statement on Sunday that Butina had been transferred from a prison in Washington to one in Virginia on Friday without notice or explanation and that her personal belongings had also been confiscated.

She had been subjected to a strip search and shackled during the transfer, it alleged, before being held in a cell for 12 hours without food and with the lights on. She now faced a solitary confinement regime, it said.

“Her situation is getting worse. It’s obvious that attempts are being made to ‘break’ Maria using additional humiliation and psychological pressure,” the embassy said.

Slideshow ( 3 images )

“We have more and more questions for the U.S. justice system. Should Maria really be doomed to suffer such borderline torture before a court verdict on the allegations against her? This lawlessness must stop.”

U.S. federal prosecutors successfully persuaded a judge in July that Butina was a flight risk and should be jailed prior to her trial.

Russian Foreign Minister Sergei Lavrov has raised the case with U.S. Secretary of State Mike Pompeo, demanding Butina be freed.

The next hearing in her case is scheduled for Sept. 10.