Myanmar's military-backed government assured visiting top EU officials on Monday that upcoming by-elections heralding the opposition's return to mainstream politics will be democratic.

"We have taken the necessary measures so that the upcoming by-elections will be free, fair and credible," lower house speaker Shwe Mann told EU development commissioner Andris Piebalgs, speaking through an interpreter.

The April 1 polls, which will see Nobel Peace Prize winner Aung San Suu Kyi stand for a seat in parliament for the first time, are viewed as a key test of the authorities' commitment to budding reforms.

A 2010 election which swept the army's political allies to power was marred by widespread complaints of cheating and intimidation.

The opposition cannot threaten the ruling party's majority even if it takes all 48 available seats in the by-election, but a Suu Kyi win would lend legitimacy to the fledgling parliament.

Piebalgs, who was carrying an EU pledge to provide 150 million euros (almost $200 million) in additional aid to Myanmar over the next two years, later met President Thein Sein behind closed doors.

"It is sustainable, what is happening here, but let's keep our eyes open," Piebalgs said afterwards. "You feel that no one doubts the sincerity of the reforms even if they have some disagreement about what is happening now."

The commissioner, who was accompanied by German development minister Dirk Niebel, was also set to meet Suu Kyi during his three-day trip.

After nearly half a century of outright military rule, the regime has surprised observers with reforms including welcoming the opposition back into the official political arena and releasing hundreds of dissidents.

Shwe Mann, a former general who is a key figure in the new regime, acknowledged there were "still some remaining political prisoners", but added "the remaining prisoners are those who have committed criminal activities".

The European Union agreed in January to begin easing sanctions on Myanmar, lifting travel bans against the nation's leaders and pledging further action if the reforms continue.

"Once we're confident that the elections are free, fair and democratic then I'm very hopeful that the sanctions will be lifted," Niebel said after Monday's talks.