Shares of UnitedHealth Group are surging before the market open Friday as Warren Buffett's Berkshire Hathaway disclosed that it recently purchased shares of the beleaguered insurer.

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

Warren Buffett's Berkshire Hathaway revealed a new stake in troubled insurer UnitedHealth, according to a regulatory filing. The Omaha-based conglomerate bought more than 5 million shares in the health care firm for a stake worth about $1.