YANGON (AFP) -- A top United Nations envoy on Sunday hailed dramatic changes in Myanmar but said by-elections would be a “key test” of the army-backed regime‘s commitment to reform.

U.N. special reporter on human rights in Myanmar, Tomas Ojea Quintana, welcomed a “continuing wave of reforms” since his last visit in August, but said “serious challenges remain”.

“The upcoming by-elections on 1 April will be a key test of how far the government has progressed in its process of reform,” he told reporters at a press conference to outline his preliminary observations.

“It is therefore essential that they are truly free, fair, inclusive and transparent.”

He said he had been told that the use of international observers was “under consideration” for the poll, which is likely to see opposition leader Aung San Suu Kyi swept into parliament.

The democracy icon’s decision to stand in the poll is seen as a key sign of progress in the country, which was dominated by the military for nearly half a century.

A nominally-civilian government came to power last year following controversial November 2010 elections and has since surprised observers with a number of positive moves including a major release of political prisoners.

Quintana said the country must learn lessons from that poll, which was marred by widespread complaints of cheating and the absence of Suu Kyi, who was under house arrest at the time.

He said reports of “irregularities” and restrictions on political party campaign activities “should be addressed seriously” ahead of the April by-election.

Last week, Suu Kyi was greeted by tens of thousands of supporters during her first trip outside Yangon since declaring her intention to contest the polls.

But the pro-democracy campaigner decided to postpone a major political rally in the central city of Mandalay, due to start on Saturday, because the venue offered by the authorities was too small.

Quintana, whose six-day trip included talks with the government and Suu Kyi, will produce a full report on the visit to be presented to the 19th session of the Human Rights Council in March.

The envoy, who visited three prisoners of conscience in the notorious Insein prison in Yangon, said he was told that conditions had improved, although allegations of ill-treatment continue.

“The government should release all remaining prisoners of conscience without conditions and without delay,” he said.

“This is a central and necessary step towards national reconciliation and would greatly benefit Myanmar‘s efforts towards democracy.”

Western nations are currently mulling easing sanctions, further raising hopes of an end to decades of isolation for Myanmar, but are likely to keep a close eye on progress in the ethnic areas and the upcoming by-election.