US long-dated borrowing costs hit their highest in over 20 years on Thursday, extending a selloff as holders demand greater compensation to hold government bonds given strong economic growth, high levels of debt and rising energy costs that have further fanned inflation fears. Bond markets have been under pressure for months, sending yields to multi-decade highs as the Iran war raised energy prices, growth has proved resilient and investors fret about government spending. Yields rise as bond prices fall.