Jailed Pussy Riot musician Nadezhda Tolokonnikova said Tuesday that she had been moved to a chilly isolation cell after she launched a hunger strike over what she called death threats and slave labour conditions.

Tolokonnikova said in a new letter released Tuesday evening that she was freezing in an isolation cell, despite prison authorities saying she was in comfortable conditions.

The 23-year-old philosophy student and activist declared a hunger strike on Monday, saying in a letter released to the media that prisoners had to work 17-hour days sewing uniforms and that she received death threats from the colony's deputy governor and fellow inmates.

She said Tuesday evening she had been moved to a cell that had a chilly temperature of 12-14 degrees Celsius (53 to 57 degrees Fahrenheit) in the penal colony in the central Mordovia region.

"Even five layers of clothing don't save you from piercing cold. I sit on a narrow cold bench and write," she said in the letter released by her husband, activist Pyotr Verzilov.

"When you are on hunger strike it's hard to sit and freeze all day: your body temperature is lowered and your head spins."

Tolokonnikova, who has a young daughter, is half-way through a two-year sentence after being convicted over punk band Pussy Riot's protest song in a Moscow cathedral last year.

The prison authorities had earlier denied that she was in a punishment cell, saying she had been put in isolation in a "safe place" that was reasonably comfortable because of her claims of being threatened.

The prison service insisted that Tolokonnikova's cell had a "level of comfort (that) meets legal norms".

"She has her personal things, food, books and stationery," it said.

Staff were continuing to bring Tolokonnikova food despite her hunger strike, according to the chairman of a Kremlin-linked prisoners' rights watchdog for Mordovia, Gennady Morozov.

Tolokonnikova said Tuesday she had asked not to be moved to the cell because she feared being "left one-to-one with the prison bosses."

She said she believed the prison bosses could use other prisoners to "put pressure on me and force me to stay silent, with a threat to ... kill me," she said.

In her first letter released Monday, Tolokonnikova described harrowing conditions, saying inmates were treated like "slaves" and that a culture of violence saw prison staff encouraging inmates to beat up rule-breakers.

She alleged prisoners were forced to work naked as punishment for sewing slowly and described insanitary conditions, with the women often deprived of weekly showers and having to unblock overflowing toilets themselves.

In the most serious allegation, she wrote the camp's deputy governor hinted she would be killed by inmates if she pushed for their rights.

He warned that if the prisoners worked shorter hours, they would be punished for failing to reach production targets and would take revenge on her.

"When they find out this happened because of you, then nothing bad will happen to you again -- because nothing bad happens in the afterlife," Tolokonnikova accused him of saying.

More than 26,000 people have signed an online petition asking the prison service to move Tolokonnikova to another prison colony.

"Any of us could be in this girl's position. Let's support Nadya and tell our friends about her letter," Pavel Durov, the founder of VKontakte, Russia's equivalent to Facebook, wrote on his personal page. "What she describes must be rooted out."

The regional Investigative Committee said it had launched a preliminary probe into her claims.

The regional prison service has denied Tolokonnikova's claims, saying prisoners work eight-hour shifts. It said that she had threatened to make her letter public unless she was allowed to move to a different job in the colony.

The other imprisoned Pussy Riot musician, Maria Alyokhina, 25, has also complained of rights abuses at her camp in the Urals region and has spent much of her sentence in voluntary isolation.

Both women have had two requests for parole turned down.

AFP