In a press briefing on Friday, he called on the Burmese government to make good on promises of political reforms, and expressed in explicit terms ASEAN's increasing disappointment over the slow pace of movement towards democracy.
The Association of South East Asian Nations (ASEAN) admitted Burma in 1997, hoping to coax it along the path to democracy. But the government of Burma - also known as Myanmar - have continually rebuffed calls since then for the release of political prisoners and the calling of general elections.
"ASEAN members including Thailand expressed a degree of frustration regarding the slowness of the process in Myanmar. So there is a feeling - and we share that feeling - that time is not unlimited," said Kantathi. "We would like to encourage Myanmar to have concrete development towards national reconciliation and democracy."
He referred to the Burmese government's refusal to release opposition leader Aung San Suu Kyi from house arrest. Last month, Aung San Suu Kyi's detention was extended for a further year, despite a personal appeal by United Nations' Secretary-General Kofi Annan for her release. She has spent much of the past 17 years under detention.
"I was hoping that she would have been released right after the term of the last detention order had expired. So I was disappointed, and I think many members of ASEAN share the same feeling," he said.
Kantathi urged the United Nations to take a greater role in ending the political stalemate in Burma.
Some members of parliament from ASEAN countries have called for Burma to be referred to the U.N. Security Council. U.N. Deputy Secretary-General Mark Malloch Brown said earlier this week that the council might take up the issue.
The military refused to accept a landslide opposition victory by Aung San Suu Kyi's National League for Democracy in 1990. In 2003, the government issued a so-called "road-map to democracy," including the drafting of a new constitution, but little progress has been made so far.