Proven reserves of both crude oil and natural gas in the United States declined in 1976, according to reports from the American Petroleum Institute and the American Gas Association.

The API said proven reserves of crude oil fell by 1.7 billion barrels in 1976 to 30.9 billion barrels compared with a 1.6 billion-barrel decline in 1975.

Proven reserves of natural gas fell by 5 per cent to 216 trillion cubic feet at the end of 1976 from 228.2 trillion cubic feet in 1975, according to the AGA.

Both associations defined proven reserves as those which are known and can be recovered economically.

The AGA said federal price controls on natural gas were the main reason for the decline of reserves, and it called for controls to be lifted on new gas supplies. "Current federal policies fail to provide the economic incentive to search for the develop new gas supplies," AGA president George Lawrence said in a statement.

The reports emphasize the U.S. energy problem, which President Carter is expected to focus on in a policy statement on April 20.

The API said almost 1.1 million barrels of crude oil were added to proven reserves in 1976 through the discovery of fields, extension and development of known reservoirs and revision of earlier estimates.

This was offset by 2.8 billion barrels of crude oil being withdrawn from reserves by production last year, the API said.

It was the sixth straight year of shrinking reserves reported by the API. The total has declined by 32 per cent since the industry trade association recorded a reserve peak of 46.7 billion barrels in 1970.

The API said it might be possible to produce an additional 4.3 billion barrels of oil through improved recovery techniques.