Standard & Poor's warned Wednesday that Warren Buffett's bid for Burlington Northern Santa Fe Corp. could sap the liquidity and capital position of the legendary investor's insurance operations at Berkshire Hathaway Inc., jeopardizing the company's AAA rating.

The ratings agency placed its ratings on Berkshire Hathaway on CreditWatch with negative implications following Berkshire's announcement Tuesday that it will buy Burlington Northern for $26.3 billion.

Standard & Poor's said it expects a significant part of the cash portion will come from Berkshire Hathaway's core insurance operations and the transaction will reduce the liquidity and "capital adequacy" of the company's insurance operations.

Standard & Poor's Ratings Services also placed its 'BBB' corporate credit rating and other long-term ratings on railroad Burlington Northern on CreditWatch with positive implications.

Standard & Poor's credit analyst Anita Ogbara said that at the close of the transaction, which is expected early next year, "we will assess the new capital structure and any potential parental support from Berkshire Hathaway."

She said she would most likely limit a ratings upgrade, if any, by one notch, to BBB+.

Berkshire Hathaway and Burlington Northern did not immediately return calls seeking comment. Buffett told CNBC on Tuesday that Berkshire will have $20 billion of consolidated cash after completing the Burlington deal.

The acquisition of Burlington Northern Santa Fe, the nation's second-largest railroad, would be the biggest ever for Buffett's investment company. Berkshire Hathaway owns a 22 percent stake in Burlington Northern and would buy up the rest under the deal.

It requires approval from Burlington shareholders and antitrust regulators. Continued...