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Scotland's prisons are "awash" with synthetic drugs that are linked to rising levels of violence, a European human rights organisation has said.

Prisoners told members of it was "easy" to obtain the drugs, which were soaked into letters before being smoked.

However, it could not provide any direct evidence to back up the claims.

The Scottish Prison Service said it was aware of problems highlighted in the report and that action was being taken.

The Council's Committee for the Prevention of Torture (CPT) also said facilities in Scotland's women's jail, Cornton Vale, were not suitably equipped to deal with vulnerable inmates.

They described finding "women who clearly were in need of urgent care and treatment in a psychiatric facility".

The committee said the women "should not have been in a prison environment, let alone segregated for extended periods in solitary confinement".

Their report also criticised lengthy periods of segregation for men - with some locked in their cells for 23 or 24 hours a day.

They said many of these prisoners became "institutionalised" into living in "virtual solitary confinement".

The human rights organisation visited five prisons - Barlinnie, Cornton Vale, Edinburgh, Grampian and Shotts - as well as police custody facilities.

It highlighted overcrowding, particularly at Barlinnie and Grampian - where there was a temporary tripling up of cells, with inmates sleeping on mattresses under bunk beds.

The report said: "The CPT noted the gradual rise of inter-prisoner and inmate-on-staff violence (notably in Edinburgh Prison), which was officially attributed to a combination of the increase in use of Novel Psychoactive Substances (NPS), mental health issues and organised crime."

The Scottish Prison Service (SPS) said it recognised overcrowding and NPS were a problem in prisons and that actions were being taken to address this.

It said an increase in prison violence was often linked to members of organised crime gangs clashing or seeking segregation from other inmates.

SPS spokesman Tom Fox told the ´óÏó´«Ã½'s Good Morning Scotland programme that trying to deal with the synthetic drugs issue had been "really difficult".

He said : "We haven't, fortunately, experienced the level of problems with these substances that maybe has been the case elsewhere.

"However, we have seen a rising trend in the last couple of years of people trying to introduce these substances into our prisons.

"They are very difficult to detect, although I am happy to say we seem to be making some progress in that direction.

"We are currently piloting new technology and working with the University of Dundee on new ways of trying to identify these substances as they are being introduced."