Warren Buffett is shrugging off Fitch Ratings' historic downgrade of US credit from the top AAA grade to AA+.

"There are some things people shouldn't worry about. This is one," the billionaire investor said.

The "Oracle of Omaha" told CNBC on Thursday that his company, Berkshire Hathaway, bought $10 million worth of US Treasuries on Monday and did the same exactly a week prior.

"The only question for next Monday is whether we will buy $10 billion in 3-month or 6-month," Buffett said.

That suggests that much like others on Wall Street, Buffett believe Fitch's downgrade won't have much of a long-lasting, negative impact on the economy, the government's ability to borrow, nor the safety of US Treasuries.

Treasuries are known as one of the safest assets in the world, in part because they're backed by the US government, which has never defaulted on its debt or missed a payment.

Treasuries are also denominated by the US dollar, the world's reserve currency that boasts a market that's incredibly liquid, large and highly rated.

Even after the most recent downgrade, Moody's has kept its top rating of US credit. S&P has maintained its AA+ rating on the United States after downgrading it in 2011.