The median of forecasts from 16 primary dealers - the large financial institutions that do business directly with the Fed - showed a 70 percent chance the central bank will for the third time expand its balance sheet via large-scale bond purchases.

The poll was conducted on Friday after the government said employers added 80,000 new jobs last month, short of market forecasts. The jobless rate stood at 8.2 percent.

In a Reuters poll conducted on June 20, the median forecast from 17 primary dealers predicted a 50 percent chance that the Fed would begin another round of purchases, known as quantitative easing, or QE3.

There are 21 U.S. primary dealers.

David Resler, U.S. economist at Nomura Securities, put the odds of more Fed action at 75 percent and said it could come as early as August.

"At this juncture, I think it's coming, primarily because the job market isn't improving enough to give the Fed sufficient comfort," he said.

In Friday's poll, eight primary dealers expected the Fed to launch QE3 at Fed meetings in August or September, of whom two said it would probably be in August. Twelve saw it happening by the end of the year.

The Fed will hold policy meetings from July 31-Aug. 1 and from Sept. 12-13, with Chairman Ben Bernanke scheduled to speak to reporters following the second one.

As Credit Suisse economists put it in a Friday note to clients, "the best argument for the Sept. 13 meeting is that a press conference is already scheduled....The best argument for Aug. 1 is, 'What are you waiting for?'"

Credit Suisse sees an 80 percent chance of QE3, according to the Reuters poll, with the announcement likely to come in September.

Some economists have suggested the approach of U.S. elections could dissuade the Fed from acting. However, Bernanke has repeatedly said that the Fed stands ready to do more if economic conditions worsen.

In two prior rounds of QE, the Fed bought $2.3 trillion in mortgage-related and government debt in a bid to depress borrowing costs and inject life into the economy, which grew at a 1.9 percent rate in the first three months of 2012.

The median estimate of dealers for the QE3 was for a program of $550 billion, with estimates ranging from $300 billion to $750 billion. Five respondents said they expected the Fed to focus on mortgage-related securities in an effort to push historically low mortgage rates even lower to boost home purchases and help existing homeowners refinance their loans.

Recent data has suggested the housing market, which has hindered economic recovery, was stabilizing, though analysts worry it could worsen if hiring continues to slow.

Last month, the Fed expanded its "Operation Twist" program, which involves selling short-term Treasuries it owns to buy long-dated ones in an effort to cut long-term borrowing costs.

Weaker growth around the world and the risk of a full-scale financial crisis in Europe were also sapping U.S. economic momentum, dealers said, and would likely force the Fed to act. They also cited concern about a raft of potential tax increases and spending cuts set to take effect at in 2013 unless Congress steps in to change the deadline.

"There are so many downside risks out there, including political stress at home and abroad," said Thomas Simons, money market economist at Jefferies & Co.

Even so, dealers polled saw just a one-in-four chance of the U.S. economy slipping into recession over the next 12 months. Several economies in Europe are already in recession, and economists worry that the diet of tax increases and spending cuts that many European governments have embraced will complicate efforts to return to growth.

That doesn't mean many expect the Fed to raise benchmark interest rates any time soon.

The Fed has held overnight interest rates near zero since December 2008 and has said they are likely to stay there until at least late 2014. Six dealers expect the first rate hike to come that year, while seven expect the Fed to hold off until 2015. One expected a 2016 rate hike, while two see the Fed tightening policy next year.

"If they do another round of asset purchases, they're signaling that even their late 2014 language is probably early for rate hikes," said Jacob Oubina, senior U.S. economist at RBC Capital Markets.

RBC, one of the most bearish dealers, sees a 50 percent chance of a U.S. recession over the next year and no rise in borrowing costs until the first half of 2015.

On the other hand, some expected the U.S. economy to continue to outpace those abroad, which should lead to a rebound next year.

"Our forecast (for a rate hike) is the second half of 2013," said UBS Securities economist Kevin Cummins. "We do have growth picking up into the second half of this year and into next year, but we also have core inflation picking up, or grinding a little higher."

In the year to May, core consumer prices, which exclude food and energy, were up 2.3 percent, according to the U.S. Labor Department.