UN rights experts on Tuesday called on Turkey to release a doctors' union leader held on "terrorism" charges after calling for a probe into alleged army use of chemical weapons against outlawed Kurdish militants.

The five independent experts, including the special rapporteurs on torture, on extrajudicial executions, and on protecting human rights while countering terrorism, called for Sebnem Korur Fincanci to be let go "immediately and unconditionally".

They pointed out that Fincanci's arrest at her home on October 26 was believed to be "in retaliation for her public comments calling for investigations into the alleged use of chemical weapons and associated deaths by the Turkish military."

Turkey has rejected allegations in media outlets close to the outlawed Kurdistan Workers' Party (PKK) that its army has used chemical weapons in operations in northern Iraq.

The PKK is listed as a terror group by Turkey and its Western allies.

Fincanci -- a renowned forensic medical practitioner and anti-torture expert -- told AFP last month that she had examined video footage and had only called for "an effective investigation" into the allegations.

Her arrest "appears part of a deliberate pattern of applying counter-terrorism legislation to discredit human rights defenders and organisations and interrupt their vital human rights and medical work", the experts said.

"We have documented many cases where counter-terrorism legislation and other criminal provisions have been used to harass, arrest, detain and convict civil society actors... on spurious grounds," said the experts, who are appointed by the UN Human Rights Council but do not speak for the world body.

They called on Turkey to stop using counter-terrorism legislation to intimidate rights defenders, warning that doing so undermines the rule of law and encroaches on fundamental freedoms and democratic values.

"Human rights defenders and medical practitioners' ability to speak truth to power must be protected," they insisted.

"Their role in exposing human rights violations is one of the cornerstones of democratic societies."