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And judging from U.S. officials’ public statements, there has been a marked shift in recent months, notwithstanding the administration’s continued statements of concern over ethnic violence, political prisoners and Myanmar's military ties with North Korea.

In early April, when the U.S. announced a “targeted” easing of the investment ban, officials initially spoke of promoting investment in such sectors as agriculture, telecommunications and tourism, rather than resource-based industries. But by the time Mrs. Clinton gave the details six weeks later, she was inviting American businesses to invest across all sectors of the economy, including oil, gas and mining.

That position angered activists such as Moon Nay Li, coordinator of the Kachin Women’s Association Thailand, who said atrocities by the military are continuing in her native Kachin State in northern Myanmar. She launched a petition on the Internet that has gained 125,000 signatories in five weeks, demanding legally binding safeguards to ensure that American businesses don’t become complicit in rights abuses.

“Foreign business and extreme violations of human rights are virtually inseparable,” she said by email, charging that investment projects often result in land appropriations and forced relocation of villagers.

The Obama administration has said U.S. companies would be expected to conduct due diligence and will still be barred from doing business with firms owned or operated by Myanmar's military. The administration hasn’t said whether U.S. oil and gas companies would be allowed to partner with state monopoly MOGE, a necessity for investing in the petroleum sector.

Patrick Cronin at the Center for a New American Security think tank said that excluding U.S. businesses, which are subject to rigorous American anti-corruption legislation when they operate abroad, would not help but hurt the course of reform in Myanmar, particularly when companies from Europe, Russia, China, India and other Asian countries are free to invest there.

While companies will resist calls for additional reporting requirements — that might, for example, monitor what they pay to Myanmar’s government — Mr. Cronin said that could be the trade-off for investing in a country with major human rights problems.