Treasury Secretary Donald T. Regan yesterday left the door open to the possibility of a United States government loan to Brazil to help that hard-pressed nation catch up with its payments to commercial banks and other suppliers.

At a press conference, Regan said that Brazil had "not as yet" asked the United States for direct help, "and I don't know if they will."

When pressed to say whether the United States would be receptive to such an appeal, Regan said: "Well, we'd have to see under what terms and what conditions. We did help them once before, and they are a friend of ours. But of course, I can't make any commitment."

The earlier loan was for $1.2 billion last fall from the Treasury's Exchange Stabilization Fund, when Brazil first began experiencing problems with its estimated $90 billion in debt.

Regan said yesterday that this loan--announced by President Reagan during his visit to Brazil last year--has been repaid.

The Washington Post reported yesterday that the multi-billion-dollar aid package put together less than three months ago for Brazil by commercial banks and the International Monetary Fund is in danger of falling apart. Unless further cash help is forthcoming, Brazil--now $700 million to $800 million behind--will be unable to pay all of its bills, it was reported.

Before his press conference statement here, Regan told reporters at the Business Council's annual meeting in Hot Springs, Va., that Brazil is not likely to default.

But he acknowledged that "they have a shortfall and they're trying desperately to figure out how to make up that shortfall. I think solutions to the problem will be found. They won't be easy to find, but I think they will be found," he said, predicting a resolution within 10 days to two weeks.