Philippine President Gloria Arroyo has said her country is unlikely to ratify a new Asean charter unless Burma frees the opposition leader Aung San Suu Kyi.
Ms Arroyo warned Burma after meeting Prime Minister Thein Sein
The regional group's new charter, which would commit members to promoting human rights and bolstering democracy, is due to be signed at a summit on Tuesday.
Burma's suppression of mass protests in September was widely condemned.
The United States has warned that Asean's credibility was at stake over its handling of the crisis in Burma.
The county's military government has acknowledged that 15 people died during the crackdown, when security forces fired on demonstrators and thousands of people were jailed.
Earlier on Monday, the EU formally adopted tighter sanctions against Burma in response to the crackdown, including an embargo on imports of gemstones, timber and metal, and a wider visa ban against officials.
In a statement published after she met Burmese Prime Minister Thein Sein in Singapore, President Arroyo said Burma would be committing itself to restore democracy and releasing Aung San Suu Kyi if it signed the charter.
"Those who will sign the charter agree to the objective, spirit and intent of establishing a human rights body - the full protection of human rights within Asean," she warned.
"Until the Philippine Congress sees that happen, it would have extreme difficulty in ratifying the Asean charter," she added.
The charter will fail unless it is ratified by the parliaments of all its 10 member states.
The Burmese authorities have not yet responded to Ms Arroyo, but they have already forced the cancellation of a planned address by the UN envoy to Burma, Ibrahim Gambari.
Host nation Singapore had invited Mr Gambari, but Burma said his briefing would interfere with "domestic matters" and gained the support of all members except the hosts, officials said.
Singapore has also used conciliatory language in the run-up to the summit, with Prime Minister Lee Hsien Loong describing Burma as "part of the family".
"If you make all these fierce statements and supposing we say we expel [Burma] from Asean, what difference does it make?" he told the BBC.
The warning by Ms Arroyo came shortly after the most senior US trade official, Susan Schwab, said Asean's credibility was at stake over Burma.
Protesters are trying to ensure Burma's plight is not ignored
Ms Schwab, who is in Singapore for the summit, said the regional grouping had a "special responsibility" for the country.
She said it could not be "business as usual" while the repression continued.
The proposed Asean charter would codify the bloc's rules for the first time in its 40-year history.
It will also help smooth the way for the group's aim of full economic integration.
Human rights groups have criticised it, however, saying the document does not include a mechanism for punishing countries which fail to meet their obligations.
The Association of South-East Asian Nations (Asean) is composed of Brunei, Cambodia, Indonesia, Laos, Malaysia, Burma, the Philippines, Singapore, Thailand and Vietnam.