Warren Buffett's Berkshire Hathaway Inc. said on Friday it sold about 18% of its stake in Delta Air Lines Inc. and 4% of its holdings in Southwest Airlines Co. this week, as the coronavirus pandemic drives the airline industry into perhaps its biggest crisis ever.

According to regulatory filings, Berkshire sold nearly 13 million Delta shares for about $314 million and roughly 2.3 million Southwest shares for about $74 million.

The sales were conducted on Wednesday and Thursday, the filings show. Berkshire previously owned about 11.1% of Delta stock and 10.4% of Southwest stock, according to Refinitiv data. No reasons for the sales were given.

Berkshire did not immediately respond to a request for comment sent to Buffett's assistant.

The Omaha, Nebraska-based conglomerate is among the biggest shareholders of the four largest U.S. carriers - Delta, Southwest, American Airlines Group Inc and United Airlines Holdings Inc.

Buffett has said three of the airline stakes are overseen by him, while one of his portfolio managers, Todd Combs and Ted Weschler, oversees the fourth.

Berkshire's sales were disclosed after major U.S. airlines applied on Friday for payroll grants from the U.S. Treasury to help keep workers employed.

The pandemic has punished the industry, as passengers stay home and carriers worldwide slash their schedules and ground planes. Delta projected on Friday that its second-quarter revenue would fall 90%.