Mr Pinheiro said he was "flabbergasted" to find the device

Brazilian envoy Paulo Sergio Pinheiro halted his visit three days before schedule after finding a microphone in a room where he was interviewing prisoners about their conditions.

On Wednesday Mr Pinheiro, now in Bangkok, reiterated a UN demand that Burma's ruling military junta release all its estimated 1,200 political prisoners.

"It is of essential importance that all the prisoners are released, so that they can participate in political transition," he told the BBC.

Burma has released about 600 prisoners in small batches since October 2000, after starting a reconciliation process with opposition leader Aung San Suu Kyi's National League for Democracy party.

"It is unacceptable releasing prisoners drop by drop, because this is a cruelty for the prisoners, some of whom are very old," Mr Pinheiro told BBC correspondent Larry Jagan.

Mr Pinheiro also expressed his disappointment at the slow pace of the reconciliation process.

"I didn't see any progress," he said. "We are in a very delicate position."

He added that the international community must do all it can so the situation is overcome as soon as possible.

Mr Pinheiro found the hidden microphone last Saturday, while talking to prisoners inside Insien jail on the outskirts of Rangoon.

As soon as he discovered the device - said to date back to World War II - Mr Pinheiro immediately walked out of the prison and lodged an official protest with the Burmese authorities.

Burma has been widely condemned for its use of forced labour

But he told the BBC that most of the other conversations he had with prisoners during his visit were conducted in unexpected places which could not have been prepared in advance.

"I think most of the work we have done in the past was not affected by the same tactic," he said.

Critics have often labelled Mr Pinheiro as being "soft" in his dealings with Burma's military rulers.

But he insisted that his approach was similar to that of another UN envoy, Razali Ismail - to press for international dialogue, the release of political prisoners and the formation of a roadmap for political transition, democratisation and free elections.