Geneva (AFP) - The United Nations called Tuesday for a "transparent investigation" into the death of Venezuelan opposition member Fernando Alban after Caracas said he killed himself in custody.

Alban had been jailed over accusations that he took part in an alleged, failed drone attack on President Nicolas Maduro on August 4.

Venezuela's Attorney General William Saab told state television VTV that Alban threw himself from a tenth-floor window on Monday at the headquarters of the intelligence service, where he had been in pre-trial detention.

A spokeswoman for the UN rights office, Ravina Shamdasani, told reporters the Caracas government had "an obligation to ensure (Alban's) safety, personal integrity and dignity".

"We are concerned about news of his death... We do indeed call for a transparent investigation to clarify the circumstances of his death," she added.

His First Justice party blamed Maduro's government for the death of Alban, who was arrested on Friday.

The attorney general has promised a thorough investigation.

Maduro has said the alleged August attack amounted to an assassination bid involving explosive-laden drones and his government and has accused neighbour Colombia of shielding the culprits.

The UN rights office also raised the alarm Tuesday over the state of 59 Colombians held in Venezuela for more than two years without being charged.

The group was "detained during security raids known as Operations for the Liberation of the People – OLPs – which the government said were designed to break up criminal gangs," Shamdasani said.

The Colombians are being held in squalid conditions, all packed into one cell, at La Yerguara prison in Caracas, according to the UN.

Venezuela has accused them of being paramilitaries, but Shamdasani said no evidence had been brought forward to support the charge.

They lack food, water and many are reportedly ill, she added.

In November of last year, a Venezuelan judge ruled they should be unconditionally released.

"We call on the Venezuelan authorities to comply with this ruling and free them," Shamdasani said.