MYANMAR - Army-ruled Myanmar faced more pressure on Thursday to stop the "outrageous" trial of opposition leader Aung San Suu Kyi, as legal experts called for a U.N. probe into the regime's human rights abuses.

Diplomats were allowed to meet the Nobel Peace laureate and observe her trial on Wednesday, but the rare concession failed to ease pressure even from Myanmar's usually acquiescent neighbours.

"We are happy that the Myanmar authorities let our people see Daw Suu Kyi, but it's not the end," Thai Foreign Minister Kasit Piromya said after diplomats from Thailand, Singapore and Russia met her at a prison house on Wednesday.

"Our main objective is the release of all political prisoners that will lead to national reconciliation," said Kasit, whose country holds the rotating chair of the Association of South East Asian Nations (ASEAN), which includes Myanmar.

Suu Kyi, 63, is charged with breaking the terms of her house arrest after an uninvited American intruder swam to her Yangon home this month. Her two female assistants and the American, John Yettaw, are also on trial.

Suu Kyi faces up to five years in jail if found guilty on what critics say are trumped-up charges to silence her until after a multi-party election in the former Burma in 2010. Her current detention order expires on May 27 after a spell of six years.

The special court held inside Yangon's notorious Insein Central Prison was again closed to the media and public on Thursday, a day after 30 diplomats and 10 Burmese journalists were allowed to attend a 45-minute hearing.

Suu Kyi, who was treated recently for low blood pressure and dehydration, appeared healthy and confident and told the visitors she hoped to see them "in better days".

State-run MRTV broadcast footage of her meeting the three diplomats. It also showed what appeared to be prison staff quarters where Suu Kyi is being held.

"OUTRAGEOUS" TRIAL

In Washington, Secretary of State Hillary Clinton said the upcoming election would be illegitimate because of the treatment of the charismatic leader of the National League for Democracy.

"It is outrageous that they are trying her and that they continue to hold her because of her political popularity," Clinton told a congressional hearing.

"It's our hope that this baseless trial will end with a quick release of her and ... a return to some political involvement, eventually, by her and her party," she added.

In 1990, the generals refused to hand power to Suu Kyi's National League for Democracy (NLD) after it won a landslide election victory, and launched a crackdown on the party.

Suu Kyi has been detained for more than 13 of the past 19 years, most of them at her home in Yangon, guarded by police, her mail intercepted and visitors restricted.

The United States renewed sanctions against the regime after Suu Kyi was charged under a draconian security law a week ago.

European governments were weighing tougher sanctions and planned to lobby their Asian counterparts at meetings in the region next week to use their influence on the regime.

Analysts say the meetings in Hanoi and Phnom Penh were likely to produce more words than action.

Although ASEAN warned the "honour and credibility" of its troublesome member was at stake, it has consistently opposed sanctions in favour of engaging with the generals.

In Cambodia, a group of MPs said Suu Kyi's trial was a test of whether the ASEAN charter "has teeth and can be used effectively to promote peace and stability in the region".

Critics fear the charter's proposed human rights body will be too weak to sway a regime that has survived sanctions and ignored countless ASEAN and U.N. envoys seeking to broker a dialogue between the junta and opposition.

The U.N. Security Council faced new calls for an inquiry into rights abuses in Myanmar, similar to those conducted for atrocities in Darfur, Rwanda and Yugoslavia.

Systematic abuses "strongly suggest Burma's military regime may be committing crimes against humanity and war crimes prosecutable under international law," said a report by five prominent international jurists.

They wrote the "forced displacement of over 3,000 villages in eastern Burma, and widespread and systematic sexual violence, torture, and summary execution of innocent civilians" justified a U.N. inquiry into crimes against humanity and war crimes.