Warren Buffett's Berkshire Hathaway is investing billions in airline stocks

Warren Buffett's Berkshire Hathaway upped its shareholdings in various airlines during the fourth quarter, according to a regulatory filing published Tuesday.

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

Meanwhile, 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.

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 million. Wells Fargo, Coca-Cola, IBM, and American Express also remained top holdings.

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.