The Carter administration yesterday reaffirmed its commitment to the new government program to encourage American business exports, but warned that it will take a long time for the program to have a major impact.

Frank A. Weil, assistant secretary of Commerce for domestic and international business, told the Senate Commerce Committee that to "say the program will have much of an effect in the short term would be an exaggeration."

He noted that the problem of declining U.S. exports was 25 years in the making."

Weil noted that the nation's major competitors - West Germany and Japan - export a much bigger portion of their total production than does the United States.

The United States sells about 6 percent of its gross national product to foreigners, while Japan exports 14 percent of its GNP and West Germany exports 23 percent of its total production, Weil said.

Last Tuesday President Carter announced a program to stimulate American exports by increasing the amount of money the Export-Import Bank has to loan, reducing domestic barriers to exports and helping small and medium-sized businesses with foreign marketing programs.

Assistant Treasury Secretary C. Fred Bergsten told the committee that the program is an "important component" of the administration's overall plan to strength the U.S. dollar.

In large part because of the sizeable U.S. trade deficit, foreigners hold more dollars than they want, helping depress the value of the dollar in terms of other currencies, especially the West German mark and the Japanese yen.

Because the declining dollar makes imports more expensive, it has contributed to the inflation rate here. At the same time, however, the declining dollar should make imports less attractive here and make American exports more attractive to foreigners.

Bergsten said in part because of the declining dollar - and in part because other economies are growing faster than this country's increasing their appetite for U.S. products - the nation's trade balance has been improving in recent months.

Besides the export-encouragement program, Bergsten said an energy policy to reduce oil imports, a strong anti-inflation program and the Treasury's sales of gold are elements of the administration's broad program to strengthen the dollar.

A spokesman for the National Association of Manufacturers told the committee that the organization strongly supports the President's new program.