WASHINGTON (AFP) -- After a landmark visit by Myanmar's leader, the United States is looking to a new phase of greater cooperation to encourage reform with the former pariah state as it runs out of major symbolic steps.

President Thein Sein, a former general who startled observers by ushering in a wave of change when he took office in 2011, met with President Barack Obama on a four-day trip that would have been unimaginable just a few years ago.

In a sign of growing normalization, Senate Republican leader Mitch McConnell, who long spearheaded economic sanctions on Myanmar, said he would let them lapse because a renewal would be a "slap in the face" to reformists.

The Obama administration, which launched a diplomatic drive in 2009 aimed at a new relationship with the country formerly known as Burma, has already waived most sanctions as part of an "action-for-action" approach of rewarding reforms.

"The administration supports measures that create greater economic opportunity and illustrate to the Burmese the benefits of continuing reform and strengthening civil society," State Department spokesman Patrick Ventrell said after McConnell's announcement.

But both lawmakers and the administration support keeping a ban on gems, seen as a key source of funding for the military which has long played a dominant role in Myanmar and is accused of widespread abuse.

Officials acknowledge that far more needs to be done in Myanmar including improving the plight of minorities such as Rohingya Muslims, who were targeted in recent violence allegedly abetted by security forces.

Some human rights activists charge that a greater normalization with Myanmar is premature before 2015 when the country is set to hold elections, when the military may have to consider ceding true power.

Opposition icon Aung San Suu Kyi -- who has become a member of parliament under the recent reforms -- was put under years of house arrest after her National League for Democracy swept elections in 1990.

A senior US official, speaking on condition of anonymity, said that the administration was likely to speak less of an "action-for-action" approach and instead termed the policy as "calibrated."

"There's not a specific quid pro quo roadmap, but it's quite understood that we need, on our side, to see further progress before we are in a position of a completely normalized relationship," he told AFP.

Despite warming ties and a historic visit to Myanmar by Obama in November, the United States has notably balked at a substantive expansion of military cooperation.

"Our policy of easing sanctions has not been any reflection of a determination that this is 'mission accomplished,'" the official said.

"This is the beginning of a very difficult, challenging road ahead for the country, but we're now in a much better position to help with that process, with assistance and with a much more collaborative approach," he said.

McConnell voiced concerns that keeping sanctions on the books would put the United States at a disadvantage to the European Union and Australia, which have ended virtually all trade restrictions. Asian nations long traded with Myanmar.

Just Thursday, the State Department announced the finalization of requirements for US companies that make major investments in Myanmar to report on compliance with human rights and other standards.

US companies from industrial conglomerate General Electric to hotel chain Best Western to emblematic US beverage Coca-Cola have all announced plans to expand in Myanmar, which they see as a vastly undertapped market.

But advocacy group Human Rights Watch urged business to be cautious, saying that US reporting requirements did not address overarching concerns including the lack of an independent judiciary and tensions over land use.