Berkshire Hathaway has invested a total of $1.2bn (£961m) in four of the biggest airlines in the US — American Airlines Group, Delta Air Lines, Southwest Airlines and United Continental Holdings.

In a regulatory filing, the holding company owned by billionaire Warren Buffett disclosed that as of 30 September it owned 21.8 million American shares worth $797m, 6.3 million Delta shares worth $249.3m and 4.5 million United shares worth $237.8m.

The stake in Southwest was not included in the filing as the deal was said to have been concluded after 30 September. CNBC cited Buffett as saying: "I'd feel terrible if the story the next day was why wasn't Southwest included".

The investment comes as a surprise since the investor is known for staying away from the airline industry after his 1989 investment in US Airways proved to be a sour experience. He is said to have called the US Airways investment a mistake in almost every annual letter from 1989 to 1996.

He had also spoken against investing in airlines in 2007, saying: "The worst sort of business is one that grows rapidly, requires significant capital to engender the growth, then earns little or no money. Think airlines."

Buffett has not explained the latest investment, reportedly citing a self-imposed rule not to discuss investments made by his holding company.

George Hamlin, an analyst with Hamlin Transportation Consulting, said investing in all four airlines was unusual. "Over the past six or eight years, US airlines have focused far more on return on investment and far less on market share than ever before. What Buffett is basically saying is that the US industry has reached a position of relative stability and will behave," he was quoted as saying by the Financial Times.

While Berkshire has shied away from airline investments, it holds stakes in a few aviation-related businesses such as NetJets and FlightSafety International. While the former is in the business of selling fractional ownership in private jets, the latter is a training company, according to The Wall Street Journal.