Warren Buffett is extending his big oil bet amid the crude market wreckage.

His buying streak extended to a seventh day Tuesday, new filings show, as Buffett's Berkshire Hathaway bought another 861,000 shares of Phillips 66 as that stock bounced around three-month lows.

In total, during the seven days, Berkshire has bought almost 6 million shares of the company at an average price of $76.64 per share.

That's almost $460 million worth, a small, but growing, percentage of Berkshire's total stake that's now worth more than $5 billion.

The latest purchases were made as the stock went as low as $76.00, a bit above the levels it hit in the previous few days.

Since then the stock's performance has been somewhat erratic. On Wednesday it hit a low of $74.32, and on Thursday it went down to $74.32 before ending at $79.28, a gain of more than 5 percent on the day.

As there can be a lag of several days between purchases and SEC filings, we may find out soon that Berkshire continued to buy at the lower levels.