YANGON, Myanmar—With U.S. Secretary of State John Kerry expected to visit Myanmar for a regional summit this weekend, some U.S. lawmakers and human rights watchdogs are warning of stalled overhauls and urging reconsideration of the increasingly cozy relations between the two countries.

The pressure on the U.S. to be harsher in its dealings with the Myanmar government, analysts say, reflects a growing skepticism—in Washington and beyond—toward the depth of the country’s reforms, posing a challenge for the Obama administration, which has touted Myanmar’s democratic transition as one of its biggest foreign-policy achievements.

Last week, 72 members of the U.S. House of Representatives, both Democrats and Republicans, warned in a letter to Mr. Kerry that conditions in Myanmar “have taken a sharp turn for the worse on a number of important fronts.” They pointed to “horrific images of emaciated children” that continue to emerge from Rakhine state, where more than 140,000 mostly Rohingya Muslims remain displaced in camps after communal riots two years ago.

The U.S. lawmakers also warned of what they described as “stalled political reforms” linked to a slow and uncertain process around amending the military-drafted 2008 constitution, which gives the military a veto power over political changes and bars opposition leader Aung San Suu Kyi from assuming the presidency.

“Just as the beginning of the reform process required a calibrated reassessment of U.S. policy three years ago, recent disturbing developments call for a significant recalibration now,” their letter said.

Other groups like Human Rights Watch and PEN American Center, the free-expression advocacy group, have urged Mr. Kerry to keep human rights and freedom of expression issues high on his agenda. The Myanmar government recently sentenced four journalists and the chief executive of their newspaper to a decade in prison for a report on an alleged military-run chemical weapons facility.

Mr. Kerry will be visiting Myanmar for the Association of Southeast Asian Nations’ Regional Forum on Aug. 9, held in the capital of Naypyitaw since Myanmar chairs the bloc this year. He will also hold bilateral meetings with the Myanmar government.

The souring of sentiment is in contrast to Hillary Clinton’s celebrated visit to Myanmar almost three years ago. It was the first visit of a U.S. secretary of state to visit in almost six decades.

“Today there’s more skepticism about the reforms,” said Murray Hiebert, a senior fellow at the Center for Strategic and International Studies in Washington. Some U.S. lawmakers, he adds, “argue that Washington has gone too far too fast in relaxing sanctions, and exploring some military-to-military relations.”

In 2012, Washington reversed its harsh policy of sanctions against Myanmar, slowly lifting economic sanctions and beginning a steady process of engaging with those it once blacklisted, starting with the former generals who make up the new nominally-civilian government.

But this recent increased pressure on Mr. Kerry proves a conundrum for Mr. Obama’s administration, which has consistently credited Myanmar’s reform success here to its own efforts. In Mrs. Clinton’s recently launched book called “Hard Choices,” a chapter on Myanmar—describing in detail her 2011 visit and the U.S. efforts in engaging with the reformist government of President Thein Sein —ends with the line: “It was America at its best.” Any backsliding, analysts say, could stain this legacy and harm Mrs. Clinton’s bid for the Democratic nomination in 2016.

Those close to Myanmar’s reform process, though, continue to see U.S. support — and the support of their businesses — as a crucial tenet of country’s success, where investment is just starting to create new jobs and bolster the gross domestic product of a still-impoverished nation.

Pushing for the “resumption of the former U.S. policy of ostracism” must be avoided for reforms and economic development to continue, said Romain Caillaud, managing director of political consultancy Vriens & Partners in Yangon. Mr. Caillaud has been based in the city for more than six years.

“Western businesses have a key role to play in this process of change that started a few years ago only, and discouraging their investment in Myanmar would not serve the objectives of policy makers in Washington, or of the population of Myanmar,” he added.