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.
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.
A code of conduct would help Vietnam and the Philippines, which are both members of Asean.
The meetings come amid rising tensions between ASEAN members and China over the South China Sea.
US Secretary of State Hillary Clinton’s trip to the Asean Regional Forum in Phnom Penh next week will mark her second visit to Cambodia.