Berkshire Hathaway CEO Warren Buffett. Rick Wilking/Reuters Warren Buffett's Berkshire Hathaway upped its holdings in various airlines during the fourth quarter, according to a regulatory filing published Tuesday.

The 13F filing showed that Berkshire took a new position in Southwest Airlines, buying 43.2 million shares valued at $2.15 billion.

The company increased its stake in Delta Air Lines by 848% to 60 million shares worth $2.95 billion. Berkshire also raised its stake in American Airlines.

Buffett continued to load up on airline stocks even after calling the sector a "death trap for investors" during his 2013 annual shareholder meeting.

Berkshire hiked its ownership in Apple by 277%, buying 42.1 million shares valued at $6.64 billion.

Buffett's most valuable holding remained Kraft Heinz, and no change was made to the stake worth $28.4 billion. Wells Fargo, Coca-Cola, IBM, and American Express also remained top holdings.

Berkshire dumped nearly all its Walmart shares, cutting its stake by 89%.

Berkshire's combined portfolio gained 15% during the fourth quarter. The benchmark S&P 500 index of US stocks gained 3.3%.

Major investment firms including hedge funds are required to disclose their long positions in stocks every quarter. Because of a time lag, these positions may have changed or been closed by the time the filings are made.