(Reuters) - UnitedHealth Group <UNH.N> said it would buy a 90 percent stake in Amil Participacoes SA <AMIL3.SA>, Brazil's largest health insurer and hospital operator, for about $4.9 billion to tap into the country's fast-growing private healthcare market.

Amil founder and Chief Executive Edson Bueno and partner Dulce Pugliese will retain the remaining 10 percent stake for at least five years, reducing their stake from 70 percent.

The deal includes Brazilian tax benefits worth about $600 million, bringing the effective equity purchase price to about $4.3 billion, the companies said.

UnitedHealth and other U.S. health insurers are under pressure as the government reins in reimbursement for Medicaid and Medicare and competition intensifies among health plans serving employers.

"(Brazil's) growing economy, emerging middle class and progressive policies toward managed care make it a high potential growth market," UnitedHealth CEO Stephen Hemsley said in a statement.

UnitedHealth, the largest U.S. health insurer, said the deal is expected to slightly add to 2013 earnings per share.

UnitedHealth also said it expects third-quarter net earnings of at least $1.45 per share. Analysts on average were expecting $1.25 per share, according to Thomson Reuters I/B/E/S.

(Reporting by Esha Dey in Bangalore; Editing by Saumyadeb Chakrabarty)