Overseas investors are reviewing their investments or shunning Treasuries as rates at the short end rise above those at the longer end and make it unprofitable for holders of these bonds to hedge their currency risks.

The difference between short- and long-term bond rates, or the yield curve, has contracted in recent weeks as rising U.S. interest rates meet growing doubts the world's biggest economy may be slowing down, weighing on longer-dated yields.

And as short-term yields move higher than longer-term yields, the cost of hedging exposure to the U.S. dollar has gone up.

"There is the whole issue of hedging costs. That is the one thing that was inconsequential at the start of the year but now it is sizeable," said Paul O'Connor, head of multi-asset at Janus Henderson in London, whose firm manages $378.1 billion in assets.