Myanmar's opposition leader Aung San Suu Kyi says reforms in the country have "stalled" and warned against "over optimism".
Speaking at a press conference in Yangon on Wednesday, the Nobel laureate said Myanmar had not made any real reforms in the last two years.
She added that high-level talks last week with senior politicians and the head of the army had achieved little.
Next week Myanmar will host a summit attended by several world leaders.
US President Barack Obama will be at the summit, the most prestigious in the country's history.
Analysis: Ko Ko Aung, BBC Burmese
It is hardly surprising that Aung San Suu Kyi has expressed her frustration over the progress of change in Myanmar. Despite the relative openness, the military still dominates key decision-making. whether in negotiating peace with ethnic armed groups or changing the constitution.
The uniformed officers appointed by the military in parliament have refused to amend clauses in the constitution to allow Ms Suu Kyi to stand for the presidency. They hold a quarter of all the seats in both chambers.
Despite the setbacks, Ms Suu Kyi has said she will continue to push for democratic changes by means of parliamentary politics. That position was welcomed by the government when she reiterated her stance during a recent meeting with key players, including President Thein Sein and the chief of the military.
However, the decisions that have been made by parliament so far appear of more benefit to the ruling party and the military. This means that Ms Suu Kyi has little option but to convince the military to change its mentality - that could be the hardest job of her political career.
In her speech Ms Suu Kyi, who leads the National League for Democracy said: "We do think there have been times when the United States government has been overly optimistic about the reform process."
She noted however that the lack of significant reforms is "something the United States thinks about very seriously as well".
After the 2010 general election Myanmar's President Thein Sein initiated a series of reforms that resulted in the Suu Kyi-led pro-democracy opposition rejoining the political process.
Ms Suu Kyi was elected to parliament in 2012 after spending years under house arrest.
She is not the first person to criticise the slow pace of reform.
Last week the UN special rapporteur on human rights in Myanmar, Yanghee Lee, told the general assembly that while progress had been achieved, there were also signs of backtracking, citing unresolved ethnic conflicts, the incarceration of political prisoners and violence in Rakhine state.
In 2012 violence broke out between Buddhists and Rohingya Muslims, killing about 200 people in Rakhine. Since then tens of thousands of people have been displaced, mostly from the minority Rohingya community.