Myanmar's parliament reopened with Aung San Suu Kyi refusing to take a seat on Monday, while the nation's president vowed 'no U-turn' on reforms as the EU prepares to suspend sanctions.

Suu Kyi's party has refused to swear to 'safeguard' an army-created constitution in the first sign of tension with the government since a landmark by-election this month saw the democracy icon win a parliamentary seat.

The spat comes as European Union nations are preparing to suspend most sanctions against the impoverished nation for one year to reward a series of dramatic reforms since direct army rule ended last year.

Myanmar, long-isolated under military dictatorship, has seen a rapid improvement in relations with the international community after the Nobel Peace Prize winner and her party achieved a decisive win in the April 1 polls.

Suu Kyi has shown increased confidence in the reformist government of President Thein Sein in recent weeks, calling for the EU sanctions suspension and planning her first international trip in 24 years.

Thein Sein, who is currently on a visit to Japan, on Monday vowed that he would not backtrack on the country's democratisation.

"There won't be any U-turn," Thein Sein said, according to the Mainichi Shimbun.

But he rejected the suggestion that he would alter Myanmar's parliamentary oath to accommodate Suu Kyi, telling reporters that it was up to her whether or not she took the seat.

Suu Kyi's National League for Democracy (NLD) -- the main opposition force after securing 43 of the 44 seats it contested in the by-elections -- has appealed to the president directly over the stalemate, asking that the wording of the oath be changed from 'safeguard' to 'respect' the constitution.

"I don't know the circumstances of the president's remarks," NLD spokesman Nyan Win told AFP in response to reports of Thein Sein's comments. "We haven't got any formal reply yet."

The NLD, which boycotted a controversial 2010 election, agreed to rejoin the political mainstream last year after authorities changed a similar phrase in party registration laws.

Suu Kyi has said one of her priorities as a politician is to push for an amendment of the 2008 constitution, under which one quarter of the seats in parliament are reserved for unelected military officials.

A session of the lower house, which was supposed to see Suu Kyi make her debut as a politician, began without the former political prisoner and 36 other members of her party on Monday, according to an AFP photographer. MPs from other parties took the oath in a session of the upper house earlier in the day.

Myanmar analyst Aung Thu Nyein, of the Vahu Development Institute, said the NLD had "made the wrong move" in making a stand over the oath.

"I think the NLD should participate in the parliament and then they should propose comprehensive political and economic reform strategies," he said, adding that the wording appears in the constitution itself and the president is therefore unable to change it.

Myanmar's new regime has freed hundreds of political prisoners and signed tentative peace deals with a number of rebel groups as part of its reform programme, although fighting continues in the far north.

The international community has begun easing sanctions as it looks to balance fears over the sustainability of the changes and a desire to bolster regime reformers who may face pressure from those wary of change.

EU foreign ministers meeting in Luxembourg are set to approve the suspension of sanctions against almost 500 individuals and more than 800 firms in a move that could open up the resource-rich Southeast Asian nation to European firms.

The bloc also has an asset freeze and travel ban against nearly 500 individuals, but it lifted visa bans on 87 top Myanmar officials, including President Thein Sein in February.