The United States said on Tuesday Myanmar's failure to amend a military-drafted constitution raised questions about the credibility of reforms, but did not go so far as to say it would undermine the legitimacy of upcoming elections.
Myanmar emerged from 49 years of military rule in 2011 and its semi-civilian government has carried out wide-ranging democratic reforms, including freeing political prisoners and allowing the formation of political parties.
But concern is growing that the reform program is stalling or even sliding back.
Opposition leader Aung San Suu Kyi, barred by the 2008 constitution from becoming president, told Reuters last week boycotting the parliamentary elections, expected in November, was an "option" if the charter was not changed.
The constitution reserves one quarter of parliament and key cabinet posts for the military, giving it an effective veto over politics, and bars presidential candidates with a foreign spouse or child. Suu Kyi's late husband was British, as are her two sons.
The U.S. embassy said the constitution should be amended to allow civilian control of the military and provide "the right of citizens to elect freely the leaders of their choice."
"Failure to amend the constitution will raise questions about the credibility of democratic reform going forward," an embassy spokesman told Reuters.
But the embassy did not link constitutional change to the legitimacy of the elections.
"Ultimately, however, the viability and legitimacy of the 2015 election as a democratic exercise will be determined by the people of Burma."
In an interview with Reuters last week, Suu Kyi accused the United States and others of being too soft on President Thein Sein's "hardline regime."
She said too much praise had made the government "complacent" and it was backsliding on promised reforms.
The U.S. embassy spokesman said the reform process had been a mixed bag.
"We recognize the reforms undertaken over the past three years to open up the country's politics, economy and society," he said. "But reform has been inconsistent in practice, and has clearly not kept pace with popular expectations."
Amending the constitution to remove the military's role in politics or allow Suu Kyi to be president would not be easy.
Changes require a 76 percent majority vote in a parliament dominated by military members and their allies.
Even if that were to succeed, those amendments would need to be put to a national referendum run by the Election Commission, which is overwhelmed with preparations for the elections.