Jan. 16 (Bloomberg) — UnitedHealth Group Inc., the biggest U.S. health insurer, said government cuts to Medicare may make it harder to increase earnings in 2015, sending shares down the most in three months.   Rate cuts that may reach 7 percent, following a similar reduction for this year, could be "extraordinarily disruptive" to the program, Chief Executive Officer Stephen Hemsley told analysts on a conference call today.   Medicare Advantage, insurers' private version of the U.S. government program for the elderly, has been a key source of growth for the industry and provided about a fifth of UnitedHealth's profit last year. The companies' reimbursements are being scaled back to help pay for the insurance expansion under the Patient Protection and Affordable Care Act, the U.S. health-care overhaul known as Obamacare.

Insurers are facing "unprecedented uncertainty from the ACA implementation and the 2014 Medicare Advantage rate cuts," said Brian Wright, a New York-based analyst at Monness Crespi Hardt & Co., in an e-mail.