The UN rights chief Volker Turk has urged Bangladesh Prime Minister Sheikh Hasina to release BNP Chairperson Khaleda Zia for urgent medical treatment outside the country.

Turk, the UN High Commissioner for Human Rights, said in a letter to Hasina that Zia's release would be seen as "an important step towards political dialogue and reconciliation".

"I appeal to your government to consider her release to enable her to receive the urgent and specialised medical care that she requires outside of the country," he wrote in the letter dated November 1 and seen by AFP.

Bangladeshi doctors have said that Zia, a two-times former prime minister, was at "high risk" of dying without overseas medical intervention abroad after the Hasina government rejected pleas to let her leave.

Zia, 78, has been living under house arrest since her release on bail from a 17-year prison sentence in 2020.Zia has advanced liver cirrhosis, diabetes and heart problems. Three US doctors carried out minor surgery on her last month, but the government rejected a family request to allow her to travel to Germany for a liver transplant.

The country is gearing up for national elections due in January. Turk also asked the government "to prevent any arbitrary detention including detention that could be perceived as politically motivated or which might impact the environment for fair and credible elections".