Gambari is expected to discuss a planned referendum in May on a new constitution that is a key stage in what the military government calls its "road map to democracy", leading to eventual elections in 2010.

But the visit comes amid growing concerns from pro-democracy groups that the military is tightening its grip on power.

"The government don't appreciate or understand the language of compromise" Aung Zaw, Editor Irrawady magazine

Aung Zaw, the editor of Irrawady magazine based in neighbouring Thailand, said he doubted there would be any progress from Gambari's latest visit.

"The government don't appreciate or understand the language of compromise," he told Al Jazeera.

Analysts say the new draft constitution continues to give the military a leading role and veto power over legislators, as well as bars opposition leader Aung San Suu Kyi from running in an election because she was married to a foreigner.

'Frustrated'

Pro-democracy groups say they see little hope for progress from Gambari's visit [EPA]

The UN has said the itinerary for Gambari's visit remains unclear while the envoy himself recently said he had been "frustrated" at the lack of tangible results from his previous visits.

Gambari visited Myanmar two days after September's crackdown and pleaded for the ruling generals to release protesters who had been arrested, some of whom were subsequently freed.

Later in the year, however, the government expelled the senior UN diplomat in the country, just one day before Gambari arrived on his second visit.

Nonetheless, Gambari did establish a process for talks between a government appointed liaison minister and the opposition.

In a statement ahead of his latest visit, the UN said Gambari "hopes to stay as long as necessary" in Myanmar and to meet "all the groups he was not able to see during his last visit".

It added that his itinerary was still under discussion with the government.

On his last visit, Gambari was shunned by Myanmar's leader, Senior General Than Shwe.

The military also rejected his proposal for a three-way meeting with Aung San Suu Kyi, although it did allow Gambari to meet privately with her.

The Nobel Peace laureate has been held under house arrest for most of the past 18 years.

Her party, the National League for Democracy, won a landslide victory in Myanmar's last elections in 1990, but the military which has ruled the country since 1962 refused to recognise the result.