ANKARA, Turkey (AP) — A top U.N. official has expressed concerns over increased allegations of torture and ill-treatment in police custody in Turkey.

Nils Melzer, the U.N. special rapporteur on torture, said Tuesday he was alarmed by allegations that people detained for suspected links to U.S.-based Muslim cleric Fethullah Gulen or to outlawed Kurdish rebels were "exposed to brutal interrogation techniques" including beatings, electrical shocks and threats.

A U.N. statement quoted Melzer as saying: "no serious measures appeared to have been taken by the authorities to investigate these allegations or to hold perpetrators accountable."

There was no immediate response by the Turkish government which in the past declared commitment to a policy of "zero tolerance" to torture.

Turkey accuses Gulen of masterminding a failed coup in 2016 — an allegation the cleric denies.