"I have not had the latest report but I expect the Myanmar Prime Minister Thein Sein to attend the ASEAN summit," he said at a news conference. "This is a very significant step forward."

Lee said the summit marks the first time that a U.S. president will be holding a formal meeting with all 10 ASEAN leaders.

The meeting will be held on the sidelines of a summit of leaders from the 21-member Asia Pacific Economic Cooperation forum to be held in Singapore on Nov. 14-15, which U.S. President Barack Obama is expected to attend.

"It's a good sign because it means the U.S. has shifted its position and is now moving to engage Myanmar," he said at the news conference on the upcoming APEC summit.

"Ostracizing Myanmar and cutting it off altogether is not a constructive way forward, it is unlikely to yield any results," he said. He added that engaging Myanmar "isn't going to yield any immediate results either because this is a complex situation, there is no easy solution in Myanmar."

ASEAN official sources said the U.S.-ASEAN summit is slated to be held on Nov. 15, as the first full-fledged U.S.-ASEAN summit. In the past, such meetings between the two sides on the sidelines of APEC leaders' summits only involved some ASEAN countries who belong to APEC meeting the U.S. president.

In 2007, Singapore had planned to hold a summit between then U.S. President George W. Bush and all 10 ASEAN members, but the plan was not realized then mainly due to continued tension between the United States and Myanmar on the issue of democracy and human rights.

Lee said it is vital to give support to Myanmar in its efforts to move towards democracy.

"They are embarked on a course of what they call the seven-step roadmap to democratization...and I think we should encourage them along this track."

The U.S.-ASEAN summit involving all 10 members will be held amid signs of improving ties between the United States and Myanmar. Obama is trying to engage Myanmar's reclusive military regime, sending two of its top envoys to Myanmar for talks with the ruling junta and democracy leader Aung San Suu Kyi, recently.

Lee said he expected the issue of Suu Kyi's continued detention to be discussed during the summit.

"ASEAN's view is clear. We have always said that we believe she ought to be released, and I am sure this will be discussed in the U.S.-ASEAN summit too, and I'm sure both sides will state their views," he said.

APEC's 21 member economies are Australia, Canada, Chile, China, Hong Kong, Japan, South Korea, Mexico, New Zealand, Papua New Guinea, Peru, Russia, Taiwan, the United States, and seven ASEAN members, namely, Brunei, Malaysia, Indonesia, Singapore, Thailand, the Philippines and Vietnam. The three other ASEAN members -- Cambodia, Laos and Myanmar -- are not part of APEC.