US diplomats visit
Obama's call follows a visit earlier this month by two senior US diplomats to Myanmar, where they held talks and had a private meeting with Aung San Suu Kyi.
It was the highest-level US visit to Myanmar in 14 years.
Aung San Suu Kyi has spent 14 of the past 20 years under detention by the military government.
Western governments have avoided direct contacts with leaders of Myanmar for decades, citing the regime's poor human rights record and suppression of democracy.
A joint statement issued after the summit - the first ever between a US president and the Association of Southeast Asian Nations (Asean) - devoted a paragraph on Myanmar, a major obstacle in relations between the two sides.
But the statement did not call for the release of political prisoners, including Aung San Suu Kyi, only urging Myanmar to ensure that the elections it intends to hold in 2010 are "conducted in a free, fair, inclusive and transparent manner".
Thein Sein did not address leaders' concerns about Aung San Suu Kyi, said Najib Razak, the Malaysian prime minister.
"We expected a bit more, but it was not forthcoming. We hope [democracy] ... in Myanmar will become a reality sooner than later," he told reporters.
He said a reference to Aung San Suu Kyi was not included in the statement because there was no consensus.
Meanwhile, leaders at the annual Asia Pacific Economic Co-operation summit, where Obama made the call for Aung San Suu Kyi's release, have vowed in a final statement to reject trade protectionism and pursue a new strategy for growth after downturns.
The statement on the last day of talks in Singapore on Sunday said that Apec, the acronym by which the grouping is commonly known, rejects "all forms of protectionism".
|An agreement on emission reductions was
dropped from the final statement [AFP]
Several Apec leaders have been critical of the imposition of trade restrictions by Washington recently.
The statement also stated that Apec nations would work for an "ambitious outcome" at next month's crucial climate-change talks in Copenhagen, the Danish capital.
However, no final binding agreement on targets was reached.
"We ... reaffirm our commitment to tackle the threat of climate change and work towards an ambitious outcome in Copenhagen," the statement said.
Veronica Pedrosa, Al Jazeera's correspondent at the summit, said a political deal would be backed rather than binding climate change targets.
A caveat pledging to halve greenhouse gas emissions by 2050 set in an earlier draft was dropped from the final statement.
Mike Froman, the US deputy national security adviser, had said earlier: "There was an assessment by the leaders that it was unrealistic to expect a full, internationally legally binding agreement to be negotiated between now and when Copenhagen starts in 22 days."
Obama was one of 21 heads of state in attendance, along with Hu Jintao and Gloria Arroyo, the Chinese and Philippine presidents respectively, and Kevin Rudd, the Australian prime minister.
After the Apec summit, Obama flew to China, arriving late on Sunday for a three-day visit that will see him touring Shanghai, the country's financial centre, and Beijing, the capital.
Lars Loeke Rasmussen, the Danish prime minister, made a surprise visit to meet leaders on the sidelines of the summit.
Froman said.: "There was, I'd say, a general consensus of support for what prime minister Rasmussen laid out, which is... 'one agreement, two steps' where Copenhagen would be the first step in a process towards an internationally legally binding agreement."
He said Rasmussen told the meeting that "in Copenhagen, he would seek to achieve a politically binding agreement that covered all the major elements of the negotiations, including mitigation, adaptation, technology, and finance".
"I think there was widespread support among the leaders that it was important that Copenhagen be a success, that there would be the achievement of real, concrete progress in Copenhagen with operational impact," he said.