WASHINGTON - US President Barack Obama Thursday eased investment curbs on Myanmar but maintained sanctions against former junta members, seeking maximum leverage to encourage a "nascent" reform drive.

Obama's move followed calls from business and political figures in the US, Europe and Asia to lift sanctions, and warnings by democracy icon Aung San Suu Kyi against excessive optimism over a political opening. "We will license certain types of investment in financial services and for businesses to do business in Burma," a US official said. "We will continue to sanction individuals associated with the former regime."

"It is a recognition of progress, it is a recognition that opening up greater economic engagement between our two countries is important to support reformers," the official said, speaking on condition of anonymity.

The decision could usher in the first major trade and investment between the United States and Myanmar for years, and help pry open the country's backward economy, that has been left behind by speeding Southeast Asian development.

The official added that the United States would maintain wider sanctions on Myanmar as leverage to promote change and to give it the legal authority to move quickly should it detect "backsliding" by the government.

The US government will advise companies on where and whether to invest in Myanmar and would maintain restrictions on investment with the military, which has deep commercial interests in the country's economy.

Earlier, Obama said Myanmar had made progress in a number of areas including by releasing political prisoners, pursuing cease-fire talks with ethnic groups and by opening dialogue with Aung San Suu Kyi's National League for Democracy.

"Burma has made important strides, but the political opening is nascent, and we continue to have concerns, including remaining political prisoners, ongoing conflict, and serious human rights abuses in ethnic areas," he said. "I have determined that it is necessary to continue the national emergency with respect to Burma and to maintain in force the sanctions that respond to this threat," Obama said. US law currently requires the president to restrict imports from Myanmar, which for decades was ruled by a army junta, and bans US investment and export of financial services to the country.

It also blocks property and assets of certain members of the Myanmar ruling class and senior officials linked to the former junta.

President Thein Sein surprised many US observers by initiating political reforms designed to break Myanmar's isolation, and elections this year led to the NLD securing 43 of the 44 seats it contested in parliamentary by-elections.

But it is still a minority influence in parliament with one quarter of the seats in both chambers reserved for unelected military officials.

Myanmar says it has now released more than 500 political prisoners but the US and human rights groups say more needs to be done.

Hillary Clinton made the first visit to Myanmar in 50 years by a US secretary of state in December, and the two countries are moving towards exchanging ambassadors.

Obama's announcement was published hours before talks at the State Department between Clinton and Myanmar Foreign Minister Wunna Maung Lwin.

It also took place on the eve of the G8 summit at Camp David, Maryland, which Obama will host and which is likely to include discussion about how to promote reform in Myanmar.

Suu Kyi, sworn in May 2 as a member of parliament after spending most of the past two decades under house arrest, spoke to a gathering of US politicians and rights advocates including ex-president George W. Bush, via Skype this week.

"I am not against the suspension of sanctions as long as the people of the United States feel that this is the right thing to do at the moment. I do advocate caution, though," she said.

She warned that she felt sometimes that "people are too optimistic about the scene in Burma. You have to remember that the democratization process is not irreversible."

The views of Suu Kyi, the 1991 Nobel Peace Prize winner, are considered critical to any US decision to lift decades worth of sanctions on Myanmar.

US companies have been eager to enter Myanmar, fearing Asian and European competitors will seize the growing market.