(Reuters) - Proxy advisory firm Institutional Shareholder Services (ISS) has recommended Sprint Nextel Corp shareholders vote for SoftBank Corp's revised offer, after the Japanese mobile operator sweetened its buyout terms.

SoftBank said on Tuesday it agreed to raise its offer for the U.S. wireless carrier to $21.6 billion from $20.1 billion as it fights off a counter bid by Dish Network Corp .

"As terms have improved, and no other firm competing bids are currently available to shareholders, ISS continues to recommend shareholders vote for the transaction," the firm said late on Tuesday.

Earlier this month, ISS said that Sprint shareholders should vote for SoftBank's proposed $20.1 billion purchase of the No. 3 U.S. mobile provider, potentially dealing a blow to a $25.5 billion rival bid from Dish.

(Reporting by Chandni Doulatramani in Bangalore; Editing by Stephen Coates)

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