NAY PYI DAW, Myanmar — Secretary of State Hillary Rodham Clinton arrived here on Wednesday to measure the depth of the political and economic opening the country’s new government has unexpectedly begun.

After years of abysmal relations between the United States and Myanmar, the Obama administration has promised to respond to progress — Mrs. Clinton’s trip being the most significant reward so far — even as it presses for more significant steps to end the country’s repressive rule and international isolation. 

Those include freeing hundreds more political prisoners, an end to often violent repression of democracy advocates and ethnic groups, and clarification of the country’s illicit cooperation with North Korea on developing ballistic missiles and, possibly, nuclear technologies. 

Mrs. Clinton, speaking in Busan, South Korea, before flying here, said that the United States hoped that initial steps toward what President Obama has called flickers of progress would “be ignited into a movement for change that will benefit the people of the country.” 

“I’m looking to determine for myself and on behalf of our government what is the intention of the current government with respect to continuing reforms, both political and economic,” she said. 

Mrs. Clinton is scheduled to meet the country’s new president, U Thein Sein, on Thursday here, and her aides said the two would discuss the possibility of additional reciprocal steps both countries could make to ease decades of hostilities. 

“We expect this to be a very thorough review of not only the steps that they have taken and what we expect to see in the future, but the things that the United States is prepared to do in response not only to these preliminary steps, but what might be possible if the process of reform and openness continues,” a senior administration official said. 

Mrs. Clinton’s visit is the first by a secretary of state since John Foster Dulles visited in 1955, and only the second ever. An improved relationship with Myanmar, still known as Burma by the opposition and the United States, could reshape American diplomacy in the region at a time when the Obama administration seeks to shift its geopolitical focus toward Asia, in part to manage the political and economic dominance of China. 

What additional steps, if any, the administration is willing to consider remains to be seen. Lifting the broad range of American sanctions imposed on trade with Myanmar is not yet on the agenda; that would require Congressional approval that would be likely only after far more sweeping reforms here. 

Mrs. Clinton could announce smaller steps, though, like returning an ambassador or supporting aid and international financing for the tentative economic reforms that have taken root. 

Administration officials said Mrs. Clinton first wanted to see whether Mr. Thein Sein’s government was prepared to take his own steps. Officials remain wary, disappointed that the government has not freed more of the 1,600 political prisoners still being held and that Mr. Thein Sein recently denied the existence of any of them. The senior administration official also noted that the administration’s initial efforts to engage Myanmar’s leaders in 2009 were “abysmal failures.” 

Another issue of particular concern for the United States is Myanmar’s cooperation with North Korea, and American officials have pressed the government to agree to more vigorous inspections by the International Atomic Energy Agency. 

Officials said the administration had hoped Myanmar would agree to that step ahead of the meeting of Southeast Asian Nations in Indonesia earlier this month, when President Obama announced Mrs. Clinton’s visit. 

Senator Richard G. Lugar, Republican of Indiana, welcomed Mrs. Clinton’s trip but said resolving any questions about illicit nuclear research were fundamental to improved relations. “An early goal of the tentative U.S. re-engagement with Burma should be full disclosure of the extent and intent of the developing Burmese nuclear program,” Mr. Lugar said in a statement this week.