While prices were crashing, Warren Buffett kept increasing his bet on oil (PSX)

Getty ImagesWarren Buffett.

While oil was crashing, Warren Buffett was buying up oil stocks.

Daily regulatory filings dating back to January 6 show that the Berkshire Hathaway CEO has significantly upped his stake in Texas-based energy company Phillips 66.

In the same period, oil prices have plunged to a new 12-year low.

According to the filings, Buffett’s purchases have totaled about 5 million shares in the past few days, increasing his stake in the company to about $5 billion.

Phillips 66 shares were up almost 5% in afternoon trading on Thursday, and the stock is up 4% since the end of August 2015, when Buffett first disclosed a 10% stake in the company.

Buffett last bought Phillips 66 shares in September. Oil prices have plunged about 30% since then.

Read CNBC.com’s report here.