YANGON -  UN chief Ban Ki-moon prepared Thursday for a risky visit to Myanmar amid warnings that the trip will be a "huge failure" if he fails to secure the release of detained opposition leader Aung San Suu Kyi.

Ban is set to arrive in the military-ruled nation on Friday for a two-day visit that the UN says will focus on pressing the junta to free all political prisoners, including the Nobel peace laureate, who is currently on trial.

He is due to meet junta leader Senior General Than Shwe and members of opposition parties including Aung San Suu Kyi's National League for Democracy (NLD), but there are no plans yet for him to meet her, officials said.

The 64-year-old was transferred from house arrest to prison in May to face trial on charges of breaching the terms of her detention after an American man swam to her house. She faces up to five years in jail if convicted.

Human Rights Watch said Ban should not accept the return of Aung San Suu Kyi to house arrest, instead of imprisonment, as a sign of a successful visit.

"Time and again, the UN has politely requested Aung San Suu Kyi's release, but her 'release' back to house arrest would be a huge failure," Kenneth Roth, New York-based HRW's executive director, said in a statement.

"Ban Ki-moon has offered Burma's generals a roadmap to ending their international isolation... He should make it clear that the time for stalling and playing games is over and that real change is needed now," he added.

Aung San Suu Kyi has been in detention or under house arrest for most of the time since the junta refused to recognise her party's landslide victory in Myanmar's last elections, in 1990.

Her trial is due to resume on Friday, alongside that of US national John Yettaw, and NLD spokesman Nyan Win said that he would see her later Thursday ahead of Ban's arrival in Myanmar.

"The authorities informed us that five central executive committee members of the NLD are to meet Mr Ban Ki-moon. We don't know details yet," Nyan Win told AFP.

He said the five did not include Aung San Suu Kyi, despite declaring earlier this week that any visit by Ban to Myanmar should include seeing the democracy icon.

Myanmar officials said Ban would meet Than Shwe in the remote administrative capital Naypyidaw on Friday, as well as with members of 10 political parties including the NLD, before flying back to Yangon on Saturday.

The visit is Ban's first to Myanmar since he came to urge the junta to accept international aid in the wake of devastating Cyclone Nargis in May 2008, which killed around 138,000 people.

Ban acknowledged this week that the latest trip was diplomatically risky as it coincides with the internationally condemned trial, but said that finding an appropriate time to come to Myanmar had been a challenge.

Speaking in Tokyo on Tuesday, he urged Myanmar to release all political prisoners including Aung San Suu Kyi and resume dialogue with opposition leaders.

The UN says that there are more than 2,100 political prisoners held in Myanmar. The junta handed out heavy jail terms to dozens of activists last year, many of them involved in protests led by Buddhist monks in 2007.

Critics have accused the junta of using the trial to keep Aung San Suu Kyi locked up for elections that the ruling generals have promised in 2010.

HRW said Ban should not accept "vague statements" from the regime about political reform ahead of the polls.

"There is a real danger that Burma's generals will try to use Ban's visit to legitimise the 2010 elections," said Roth, adding that the UN Security Council and regional blocs had so far "failed the Burmese people".

Myanmar has been ruled by the military since 1962 and was formerly known as Burma.