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Yields for three-month paper, the maturity offered at every Belgian short-term auction, rose to -0.594 percent from a negative 0.652 percent in April.
For six-month treasury bills, last sold in early April, yields also increased to -0.594 percent from a negative 0.637 percent previously.
Belgium's government bonds are trading at negative yields, meaning the sovereign receives rather than pays interest on its borrowings, for maturities up to six years, according to Thomson Reuters data.