US Secretary of State Hillary Clinton and Myanmar President Thein Sein were to hold landmark talks on Friday as Washington moves to ease its sanctions on the once-pariah state. The two leaders will meet in the Cambodian tourist town Siem Reap on the sidelines of a business conference, amid concerns among rights groups that the US is moving too fast in its eagerness to cash in on Myanmar's vast business potential. Washington on Wednesday gave the green light for firms to invest in the southeast Asian country, including in oil and gas, in its greatest loosening of sanctions to reward reforms after nearly half a century of military rule. The decision will please US firms eager not to miss out on what some economists expect to be a goldrush in the resource-rich nation. Asian firms have been doing business in Myanmar for years, while the European Union suspended most of its sanctions against the country in April. Myanmar on Friday said Thein Sein and Clinton were expected to discuss changes that have swept Myanmar since a quasi-civilian government replaced the military junta last year. "The meeting shows the support of the US government to Myanmar's reform process," Zaw Htay, director of the president's office, told AFP. Clinton said Thursday that Washington welcomed the changes, such as freeing democracy icon Aung Sang Suu Kyi from house arrest and welcoming her opposition back into mainstream politics. But she acknowledged concerns remained. The US move "does not mean we are satisfied that reform is complete or irreversible. Political prisoners remain in detention", she told leaders of the Asia Regional Forum meeting in Cambodia. "Ongoing ethnic and sectarian violence continue to undermine progress toward national reconciliation, stability, and lasting peace. And fundamental reforms are required to strengthen the rule of law and increase transparency." Thein Sein's comments to the UN Thursday that refugee camps or deportation was the "solution" for stateless Muslim Rohingya, following communal violence last month in western Myanmar, are likely to alarm Western nations worried about rights. Myanmar -- along with regional neighbours -- has called for all sanctions to be lifted as the country embarks on its "second wave" of economic reforms. Suu Kyi welcomed the sanctions decision, but called for greater transparency at state-owned Myanma Oil and Gas Enterprise (MOGE), which US firms will be able to do business with under the new rules. Her comments were echoed by influential US Senators John McCain and Joe Lieberman, who said operations at the organisation "remain non-transparent and the billions of dollars in foreign investment that it receives remain unaccountable to the people and parliament of Burma". Human Rights Watch went further, saying the United States had "caved to industry pressure" because it did not insist on reforms in governance and human rights. But Clinton said the rules would encourage investment "in a manner that promotes responsible development and further reform". All US companies that invest more than $500,000 in Myanmar will be required to file reports to the State Department each year that show their consideration for human rights, workers' rights and the environment. US firms remain barred from doing business with the Myanmar defence ministry or armed groups. Myanmar, left impoverished by decades of economic mismanagement and isolation under army rule, is seen as the next big frontier in Asia for firms wanting to take advantage of its resources, cheap labour force, high growth potential and strategic position between China and India. Thein Sein will be introduced by Clinton at a US-ASEAN business forum Friday in Siem Reap which will be the largest ever gathering of American corporate leaders in Asia. Executives from Coca-Cola, Caterpillar, DHL and Goldman Sachs are among dozens of US companies travelling to the conference. A high-level delegation of US business leaders will be visiting Yangon and the capital Naypyidaw in the coming days.