(Reuters) - Federal Reserve Governor Christopher Waller said on Friday it's possible that a key underlying interest rate may rise in the future after years of declines, but it's too soon to say if that will happen.

"There has been a lot of debate during the past year as to whether or not 'R-star' has increased," Waller said in the text of a speech prepared for delivery to the Reykjavik Economic Conference.

R-star is the interest rate that neither stimulates nor restricts the economy when inflation is at the U.S. central bank's target. While it's a rate that moves slowly and can't be measured with precision and is bound by uncertainty, the concept nevertheless helps explain how stimulative or restrictive monetary policy is at a given time.

In his speech, Waller noted that R-star has seen a long-term decline due to a number of factors, including strong demand for U.S. government debt in a world where global trading terms had been liberalizing, regulation changes, falling inflation and less volatile economic activity. But with demographic shifts and other forces at play, many officials have wondered if R-star will rise in the future. If it did, it would herald a new higher interest rate environment.

A key factor in the decline of R-star, Waller noted, has been higher demand for Treasury debt issuance compared to supply. But ballooning U.S. government borrowing coupled with other factors could be changing that calculus.

"If the growth in the supply of U.S. Treasuries begins to outstrip demand, this will mean lower prices and higher yields, which will put upward pressure on R-star." But he added "only time will tell how large a factor the U.S. fiscal position will be in affecting R-star."

Waller did not comment on monetary policy or the interest rate outlook in his prepared remarks.

He noted that despite some commentary to the contrary he does not see the dollar at risk of losing its preeminent status in global finance.

"Notwithstanding the drumbeat of warnings from some that the U.S. dollar is in danger of losing its primacy in global trade and finance, it remains by a very large margin the world's reserve currency," he said. "U.S. government debt, likewise, remains the primary form of low-risk asset, which is reflected in the huge stock of Treasury securities held as foreign exchange reserves around the world."

But he noted the current path of U.S. government borrowing can't be sustained indefinitely.