YANGON: A UN envoy said Friday he "deeply regretted" that Myanmar's ruling junta had refused to let him meet democracy icon Aung San Suu Kyi and called for her immediate release ahead of elections this year.

Tomas Ojea Quintana criticised the military regime as he ended his latest mission to Myanmar, a five-day trip focused on inspecting the human rights situation ahead of the country's first polls in two decades.

"I deeply regretted that my special request to meet Daw Aung San Suu Kyi was not granted," Quintana told reporters at Yangon international airport before flying to Bangkok. Daw is a Burmese-language term of respect.

"I am disappointed that even this time I was unable to meet her at this crucial time in this election year, the first national election in 20 years," said Quintana, making his third trip to Myanmar.

He was also refused access to reclusive junta chief Than Shwe and instead met Foreign Minister Nyan Win, Home Affairs minister Maung Oo and the chief justice, attorney general and police chief in the capital Naypyidaw Friday.

Quintana said that during the meetings he was given no idea of a date for the elections that the ruling generals have promised to hold this year, or even when long-awaited electoral laws would be announced.

He added that elections required the release and participation of all "prisoners of conscience" to be regarded as fair, but that the Myanmar government refused to acknowledge the existence of such detainees.

"Despite anticipation of the landmark elections I have not received any indication that the government is willing to release all prisoners of conscience," he said, adding that Suu Kyi's should be freed "immediately".

The envoy also urged the government to allow the full participation of ethnic minorities, whom rights groups say are persecuted by the regime.

Nobel Peace laureate Suu Kyi has been in detention for 14 of the last 20 years. She had her house arrest extended by 18 months in August after a bizarre incident in which an American man swan to her lakeside home.

Quintana was allowed to meet key figures from Suu Kyi's National League for Democracy (NLD) during his visit, including vice chairman Tin Oo, who was freed from house arrest on February 13 after seven years in detention.

Tin Oo said at the meeting late Thursday they had told Quintana of their request for a meeting between Suu Kyi and Than Shwe and between her and the NLD's central executive committee.

The NLD has not yet said whether it will take part in the polls, the first in Myanmar since 1990 when the NLD won by a landslide. The military subsequently annulled the result.

Myanmar's new constitution, voted through in a 2008 referendum just days after a devastating cyclone killed around 138,000 people, effectively bars Suu Kyi from standing and reserves a quarter of legislative seats for the military.

The junta has also continued a crackdown on dissent ahead of the polls.

A court at Yangon's notorious Insein prison sentenced Buddhist abbot Gaw Thita to seven years in jail on various charges on Wednesday, the opposition said, the fifth dissident to be imprisoned during Quintana's visit alone.

Four women activists were sentenced to two years each with hard labour on Monday, the same day Quintana arrived in Myanmar.

The United Nations says there are at least 2,100 political prisoners in Myanmar's notorious jails.

Myanmar, which was formerly known as Burma, has traditionally been loath to allow UN officials to meet Suu Kyi, even refusing to let UN chief Ban Ki-moon see her when he visited the country last year.

US officials have however received a warm welcome since President Barack Obama's administration began a dual track of engagement alongside sanctions.

US assistant secretary of state Kurt Campbell met Suu Kyi last year, as did US congressman Jim Webb when he visited Myanmar to secure the release of John Yettaw, the American who swam to her house.