Billionaire investor Warren Buffett has taken a $1 billion stake in Apple, whose stock has been in an extended slump.

The stake was revealed in a regulatory filing by Buffett's holding company Berkshire Hathaway after Apple shares skidded last week to their lowest level in two years.

The filing did not indicate when Buffett purchased the shares or his reasons, but Buffett is renowned as a "value" investor who takes positions in firms which trade below what he sees as their fair value.

The filing with the Securities and Exchange Commission showed Berkshire with a holding of 9.8 million Apple shares at a value of around $1.07 billion.

Apple has been under pressure as a result of slowing iPhone sales, threatening its chief revenue and profit source.

Last Thursday, the dip in Apple briefly boosted Google parent Alphabet to the number one position among companies based on market value.

Buffett's entry comes weeks after another billionaire, Carl Icahn, announced he had sold all his Apple holdings after an extended campaign to get the company to deliver more value to shareholders.

Buffett, who is among the world's richest people, controls through his holding group a number of insurance, retail and media operations.

Berkshire recently boosted its stake in IBM while reducing its holdings in Wal-Mart.

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APPLE INC.