Thailand's prime minister has expressed "concern" to his Burmese counterpart over the lack of participation by Burma's largest opposition party in the drafting of a new constitution. The opposition refuses to take part, because the government will not release the party leaders from detention.

Thai Prime Minister Thaksin Shinawatra made his concerns known Friday during a one-day visit to Thailand by Burma's prime minister, Khin Nyunt.

Mr. Thaksin told reporters he "expressed my concern" to Khin Nyunt about the continued detention of Aung San Suu Kyi and Tin Oo, leaders of the opposition National League for Democracy. But he said he was careful not to "intervene" in Burma's internal affairs.

He said Thailand was not planning on imposing trade sanctions on Burma, if Rangoon refused to release the NLD leaders.

The Burmese delegation arrived early Friday for talks covering bilateral issues, ranging from trade to the halting of drug smuggling across the border into Thailand.

But the major focus of the meeting was the constitutional convention that began in Rangoon on May 17, without the NLD present.

Foreign Ministry spokesman Sihasak Phuangketkeow later described the talks as "very frank and very candid."

"The [Thai] prime minister basically reiterated that we're ready to be supportive in any way we can to the process of national reconciliation," he said. "The basic position of Thailand is, we believe the participation of the NLD would reflect the spirit of national reconciliation in Myanmar [Burma]."

The U.N. special envoy on Burma, Paulo Sergio Pinheiro, said this week that Burma had missed a "unique opportunity" by failing to free Aung San Suu Kyi, and said the convention was meaningless without the NLD's participation.

Khin Nyunt made a similar visit to Malaysia earlier this week, where Malaysian Prime Minister Abdullah Badawi also raised the issue of Aung San Suu Kyi's detention. Mr. Abdullah said he had asked Khin Nyunt to keep the Malaysian government "informed" of the NLD leader's status.

Soon after becoming prime minister in August, Khin Nyunt, set out a "road map" to political reform in Burma, which includes the drafting of a constitution and national elections. But no time frame has been set.

The NLD won a landslide victory in national elections in 1990, but was never allowed by the military government to take power. Aung San Suu Kyi has been in and out of prison or house arrest for most of the time since.