In its monthly report, the Treasury Department said Thursday that total holdings edged down 0.1 per cent in May to $6.13 trillion after a bigger 0.6 per cent decline in April.
China boosted its holdings by 0.5 per cent to $1.27 trillion in May, but Japan, the second biggest foreign owner of Treasury debt, trimmed its holdings 0.1 per cent to $1.21 trillion.
The demand for U.S. Treasury securities, considered one of the world's safest investments, is expected to remain strong this year.
Much of the debt is owned by foreign central banks. The holdings by foreign governments rose 0.7 per cent in May to $4.16 trillion, 68 per cent of the total foreign holdings.