UPDATE 1-Vietnam fails again to sell any govt bonds

(Updates with auction result)

HANOI, May 16 (Reuters) - Vietnam's State Treasury has failed for the sixth time in a row to sell any government bonds at an auction on Friday when it sought to raise 500 billion dong ($31 million), the Hanoi stock exchange .HASTCI said.

Bidders sought yields of between 10.5 percent and 15 percent for the May 20, 2013 bonds but none of them won to buy any debt, the exchange said in a statement but gave no reasons.

At the previous auction on April 25, the State Treasury also could not sell any of its debt issue worth 1 trillion dong, the fifth failure in a row since March 7, the Hanoi stock market said.

The last time the State Treasury was successful in the bond sales was on February 28 when it raised 700 billion dong worth of two-year debt with an annual yield of 7.68 percent.

Banks, the frequent buyers of government debts, have been struggling to raise dong funds after the central bank tightened monetary conditions to fight Vietnam's double-digit inflation. ($1=16,013 dong) (Reporting by Ho Binh Minh; editing by Neil Fullick)