The United States is poised to allow U.S. companies to invest with Burma's state oil and gas enterprise as the Obama administration takes its biggest step yet to roll back sanctions, marking a rare break from pro-democracy leader Aung San Suu Kyi.

1st U.S. ambassador in 22 years 

On Wednesday, the U.S. Embassy in Burma said Washington's first ambassador to the Southeast Asian nation in 22 years had arrived. 

Embassy spokeswoman Adrienne Nutzman says Derek Mitchell flew into the main city, Yangon, on Wednesday and will formally present his credentials to President Thein Sein in the capital Naypyitaw.

Mitchell had previously served as U.S. President Barack Obama's special envoy to Burma, as known as Myanmar. The U.S. Senate confirmed his appointment as ambassador in late June.

Suu Kyi, the Nobel Peace Prize laureate who has long been the guiding force on U.S. policies toward Burma, which is also known as Myanmar, last month advised against investment by foreign companies with the state Myanma Oil and Gas Enterprise, or MOGE, because of concerns over its accountability and transparency.

Her comments reflected the growing disagreement between human rights groups and business advocates over how the U.S. should proceed in easing restrictions.

While Suu Kyi has cautiously supported suspending sanctions as a reward for Myanmar's shift from five decades of authoritarian rule, she and other democracy advocates are wary about investment in MOGE, which had been an economic lifeline for the former ruling junta.

But doing business with MOGE is the only way to gain access to Myanmar's potentially lucrative energy resources and U.S. companies fear they will lose out to foreign competitors if the restrictions aren't lifted.

Barred from investing in military-owned firms

Recognizing continuing concerns over corruption and rights abuses in Burma, the administration is expected to require U.S. companies to report on their investments in the country.

Secretary of State Hillary Rodham Clinton said in May that U.S. companies would be allowed to invest in all sectors of Burma's economy, though not firms owned or operated by the military. She also announced the suspension of a ban on the export of U.S. financial services, seen as vital for starting to do business there.

The administration is expected to take the next step this week, when it announces the issuance of a general license that finally opens the door for American firms to operate in one of Asia's last untapped markets.

Clinton is currently travelling through Southeast Asia, a trip centred on a meeting in Cambodia of the region's foreign ministers but also underscoring U.S. efforts to deepen trade and investment ties with a region of rising prosperity and importance as an export market.

An announcement on easing sanctions would also coincide with the arrival in Burma of Derek Mitchell, the first U.S. ambassador to the country in 22 years, as Washington normalizes its diplomatic relations with a former pariah state.