Officials at the Federal Reserve have tried to calm investors by emphasising that bond purchases will not halt until the economy strengthens.
The comments are the strongest signals yet by monetary officials following recent market turmoil.
Markets were rattled after chairman Ben Bernanke indicated last week that the Fed could start scaling back its extraordinary support of the economy.
A survey on Thursday showed a sharp rise in mortgage rates this week.
William Dudley, president of the Federal Reserve Bank of New York, said: "If labour market conditions and the economy's growth momentum were to be less favourable, I would expect that the asset purchases would continue at a higher pace for longer."
'Data over date'
The Fed buys $85bn (£55.8bn) a month in bonds. This is aimed at lowering long-term interest rates, which are used as a benchmark to determine the rates at which households and businesses borrow for long-term investments.
Markets saw a dramatic sell-off over the past week after Mr Bernanke said that the central bank could begin paring its bond purchases by the end of 2013 and wind them down completely by the middle of 2014.
Stock markets around the world plunged, and benchmark 10-year US bond yields jumped.
Investors took fright over concerns that the so-called "tapering" would lead to higher interest rates, which may nip the US economic recovery in the bud.
But Jerome Powell, a member of the Fed's Board of Governors in Washington, said investors appeared to have misinterpreted that the Fed would end its purchases.
"The path of rates will ultimately depend on the path of the economy," said Mr Powell in an appearance on Thursday at the Bipartisan Policy Center, a Washington think tank.
"I want to emphasise the importance of data over date."
Earlier this week Richard Fisher, president of the Dallas Federal Reserve and a member of the Fed's rate-setting committee, blamed the "feral hogs" of financial markets for pushing up the yields on US Treasuries.
"The Fed has made efforts to talk the market back from those assumptions," said Ian Lyngen, senior government bond strategist at CRT Capital Group.
But soothing comments from Fed officials seem to have had little effect.
Mortgage rates on 30-year fixed loans soared to their highest average in two years, reported mortgage buyer Freddie Mac. The rate is now 4.46%, a full percentage point more than a month ago.
Mortgage rates tend to reflect the yield on the 10-year Treasury bill, which recently hit a 22-month high.
According to Bankrate.com, a company that helps consumers compare and calculate mortgages, a buyer who locked in a 3.35% rate in early May on a $200,000 mortgage would pay $881 (£578) a month.
Today, that same mortgage would cost $1,008, more than $100 each month.