That brings to 36 the total number of Americans known to have died; nineteen of the additional victims reported Wednesday were in Thailand and the 20th in Sri Lanka, two of the hardest-hit countries, said the official, speaking on condition of anonymity.
The decision to list them as presumed dead is based on accounts of survivors, the official said.
Some 4,000 to 6,000 Americans have not been located, but the State Department has declined until now to provide any estimate of how many are presumed dead. This is in contrast to other governments that have lost people in the disaster.
On Tuesday, department spokesman Adam Ereli said there were 16 known dead, eight in Thailand and eight in Sri Lanka.
The State Department has hesitated to estimate how many Americans who have not been located were assumed to be victims. For more than a week, department officials have been checking airline passenger lists, U.S. embassies abroad and sifting through telephone calls for concerned relatives and friends.
Hundreds of names have been taken off the tentative list of Americans who had not been located, but others have taken their place as the inquiry proceeds.
CBS News Correspondent Sandra Hughes reports
Embassy officials are asking travelers to check in with families and U.S. diplomatic posts.