Warren Buffett's Berkshire Hathaway Inc disclosed a $4.48 billion stake in oil refiner Phillips 66, according to a Reuters news report this past Saturday.

The 57.98 million-share, or roughly 10.8 percent, stake was revealed in a Friday night filing with the U.S. Securities and Exchange Commission.

According to Business Insider, whose CEO & Editor-in-Chief is Henry Blodget, Buffett had been aggressively buying shares of Phillips 66 since Wednesday of this past week.

Berkshire once held a large stake in the Houston-based company, but shed nearly two-thirds of it in February 2014 when it swapped $1.35 billion of shares for a chemicals business that it folded into its Lubrizol unit, which Buffett had acquired in March 2011 for $9 billion.

Phillips 66 spokesman Dennis Nuss on Saturday said Phillips 66 does not comment on specific shareholders.

According to the above mentioned Reuters report, Berkshire may have begun rebuilding its Phillips 66 stake in the second quarter, when it bought $3.09 billion of equities overall.

In an August 14 SEC filing detailing its U.S. stock holdings, Berkshire did not mention Phillips 66, after having previously reported a 7.5 million-share stake as of March 31, but said it disclosed some information confidentially to the regulator.

The SEC sometimes lets Omaha, Nebraska-based Berkshire do this so Buffett can quietly buy a large amount of stock, without worrying about investors piggybacking on the famed investor's apparent stamp of approval.

He did this in 2013, when Berkshire amassed a $3.45 billion stake in ExxonMobil Corp. Buffett sold that stake in last year's fourth quarter.

Berkshire does not normally say whether Buffett or his portfolio managers Todd Combs or Ted Weschler make specific investments, but larger investments are generally Buffett's.