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.

Berkshire owned 5.04 million UnitedHealth shares worth about $1.57 billion as of June 30, it said in a U.S. Securities and Exchange Commission filing. The investment comes as the company's shares have been battered by rising health care costs,