Briefing reporters in Ankara, Nils Melzer, the UN special rapporteur on torture, said he had visited numerous prisons and met Turkish officials as well as individuals detained over their alleged involvement in the botched coup.

He said: "Torture and other forms of ill-treatment seem to have been widespread in the days and weeks following the failed coup," particularly at the time when suspects were detained.

Mr Melzer spoke at the end of a six-day visit to Turkey to look into torture and will present his findings in March 2018 to the UN Human Rights Council.

After visiting detention facilities in Ankara, Diyarbakir, Sanliura and Istanbul, he described overall conditions as satisfactory, but expressed concern over emergency measures such as the extension of pre-trial detention to 30 days and denying a detainee access to a lawyer for up to five days.

"Worldwide experience shows us that it is precisely in the first hours and days after arrest that the risk of abuse, including torture and other forms of ill-treatment, is highest," he said.

He urged the Turkish government to live up to its declared "zero tolerance" policy on torture.

"There is ... an environment of intimidation in Turkey that is conducive to torture and ill-treatment and the authorities - although they have a policy of zero tolerance for torture - they are not following up to investigate these allegations," he said.

The preliminary findings echo those of Human Rights Watch, which documented 13 cases of alleged abuse in an October report, and Amnesty International, which said it had it has collected "credible evidence" of torture by police in Ankara and Istanbul.

Turkish officials have dismissed allegations of torture as baseless propaganda.

"Ahead of the coup we were already receiving very serious reports of torture and ill-treatment, mostly in the south east of Turkey," Amnesty International's Turkey researcher Andrew Gardner said. "But what we saw after the violent coup attempt of July 15 was an explosion in the number of cases."

Allegations of ill-treatment and torture, he said, are being made by a wider range of people, including individuals with no connection to the coup events or the violence in the south east, which has witnessed the resurgence of a decades-long conflict between the state and an outlawed Kurdish movement.

Mr Gardner said a "climate of fear" - coupled with a state of emergency that increased pre-trial detention to 30 days and saw the dissolution of dozens of NGOS and three lawyer associations that were active in documenting issues of police ill-treatment and torture - has hindered independent documentation of abuses.

In the face of repeated allegations that individuals detained after the coup were tortured, President Recep Tayyip Erdogan has insisted his country has "zero tolerance toward torture" but officials have also dismissed the allegations of rights groups as propaganda by coup backers.