Warren Buffett's Berkshire Hathaway has acquired 5 million shares in UnitedHealth Group, a regulatory filing showed Thursday, sending the health insurer's stock up 7% in extended trading.

Billionaire investor Buffett owned about 1.18 million shares in UnitedHealth between 2006 and 2009, before selling his entire stake in 2010 amid a broader retreat from health insurers.

The investment comes as UnitedHealth faces soaring medical costs, federal investigations, the fallout of the killing of a top executive and a cyberattack last year.

The healthcare conglomerate has signaled prolonged pain with a new, far lower profit forecast as it sees billions of additional costs in the upcoming quarters.

Shares of the Dow component have slumped 46% so far this year.