Unprecedented impasse leaves some officials pointing finger at Cambodia, questioning bloc cohesiveness.
Meanwhile, China has built strong relationships across the region, in countries like Cambodia, which could divide Asean countries.
A senior State Department official said the talks will take place Friday on the sidelines of a conference in Cambodia.
China says ASEAN is not the place to resolve these dispute because it is not about the regional forum.
Four ASEAN members - Brunei, Malaysia, the Philippines and Vietnam - claim overlapping parts of the South China Sea.
Mam Sonando says he will defy a government threat of prosecution and return to Cambodia from abroad this week.
US Secretary of State Hillary Clinton delivered an address ahead of her meeting, saying she “strongly” supports Asean.
Patrick Devillers, who has been detained since his arrest in Phnom Penh on June 13, can return at his own request.
Asean leaders are expected to approve the declaration in a major summit in November.
The feeling from ASEAN officials was that Burma, should be rewarded.
Regional tensions have risen recently, with Vietnam and the Philippines accusing China of aggressive behavior in the sea.