NEW YORK/SINGAPORE/LONDON, Aug 18 (Reuters) - Persistently higher yields at U.S. Treasury auctions are increasing the cost of refinancing debt and funding future deficits, as investors demand greater compensation to absorb Washington's borrowing needs, raising questions about their appetite to continue buying U.S. debt.

Investors are concerned about the sheer volume of borrowing that must be financed in the years ahead, ‌analysts said. U.S. national debt is approaching a record $40 trillion, according to the Treasury Department, while the fiscal deficit remains large.