U.N. special envoy Ibrahim Gambari arrived in Myanmar on Thursday amid waning optimism about his mission to get the military junta to start talks with the opposition on political reform.

It will be his first chance to hold face-to-face talks with the generals since their unexpected announcement last month of a constitutional referendum to be held in May, to be followed by a general election in 2010.

"Gambari should tell the generals that marching a fearful population through a stage-managed referendum will not advance democracy or reconciliation in Burma," Brad Adams, Asia director at Human Rights Watch, said in a statement.

"A referendum under these repressive conditions will only cement in place continued military rule," he added.

It is Gambari's fifth visit to the former Burma since he was appointed in early 2006 and his third since a crackdown on monk-led protests last September.

The continued house arrest of National League for Democracy leader and Nobel peace laureate Aung San Suu Kyi, and the opposition's boycott of the constitution-drafting process, have led many foreign governments to reject the charter and election timeline as a sham.

The NLD has criticised it, but stopped short of calling on supporters to vote no in the referendum.

"Our leaders will tell him some bare essentials we would like of the referendum to be held by the regime in May when we meet him," NLD spokesman Nyan Win said on Wednesday.

Gambari's message to the junta on his previous visits has been to release all political prisoners, Suu Kyi included, and include the opposition and Myanmar's ethnic groups in the constitution-drafting process.

His requests appear to have fallen on deaf ears, as has his push to get the generals to engage in direct talks with Suu Kyi, whom Gambari is expected to meet during his visit. The rest of his itinerary remains unclear.