Ban said he had pressed top general Than Shwe on the issue when they met for the second time in two days in the bunker-like capital Naypyidaw, but was told the Nobel Peace laureate was off limits because she is on trial.

"I am deeply disappointed," Ban told reporters on the tarmac at Napyidaw airport before heading to Yangon. "It is a setback for the international community and it is a missed opportunity for the Myanmar authorities."

The refusal will add fire to critics of Ban's visit to Myanmar, which had been considered diplomatically risky because of its timing during Aung San Suu Kyi's trial on charges of breaching the terms of her house arrest.

The 64-year-old was transferred from her lakeside home to Yangon's notorious Insein prison in May to face trial after an American man swam uninvited to the property. She faces up to five years in jail if convicted.

"The senior general told me repeatedly that while he really wanted to agree to my request, at this time he felt sorry that because Daw Aung San Suu Kyi is now under trial, this would not give a good impression," Ban said.

"They didn't want to be seen as being interfered with or pressured by our side. But when the time comes he said he would consider this request."

Rights groups had warned that his visit would be considered a major failure unless he managed to win the release of Aung San Suu Kyi, who has been detained for most of the last two decades.

Ban said however that he had pressed "very hard" for the release of all political prisoners and that he expected the regime to free some ahead of elections promised by the generals in 2010.

"I believe they are very seriously considering releasing political prisoners if not soon, at the latest before the beginning of this election," he said.

Critics have accused the junta of using the trial as an excuse to keep Aung San Suu Kyi locked up for the polls.

Ban said that in his talks with Than Shwe he had "discussed elements necessary for creating conducive conditions for elections next year to make them transparent and credible."

The UN chief is now due to visit areas affected by deadly Cyclone Nargis in 2008. He made his first visit to the country after the disaster, when he managed to persuade the regime to accept international aid.

Ban would also give an unprecedented public address to diplomatic missions, UN agencies, international and non-governmental organisations in Yangon before his departure on Saturday evening, officials said.

UN officials travelling with Ban said there had been a "very lively exchange of views" after Ban proposed a five-point agenda for reforms at Friday's meeting.

There was "considerable resistance" to the proposals, including the establishment of a UN "good offices" bureau in Yangon to provide a permanent structure for Ban and his special UN envoy to Myanmar, Ibrahim Gambari.

Aung San Suu Kyi appeared in court in Yangon on Friday but the trial was adjourned for a week because the judges had not received an earlier judgement barring two defence witnesses, her National League for Democracy (NLD) said.

The case has sparked international outrage, with US President Barack Obama calling it a "show trial" and a host of world leaders and celebrities calling for her release.

She has been under house arrest or in detention for 13 of the last 19 years since the junta refused to recognise the NLD's landslide victory in Myanmar's last elections, in 1990.

Ban has faced recent criticism for his softly-softly approach to the job of secretary general, but diplomats say he had hoped his quiet brand of diplomacy would work with Myanmar's generals.

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

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