The Federal Reserve will not waver from its aggressive policy stance when one of its two bond-buying programs expires at year end, and it is prepared to do even more to get Americans back to work, two Fed officials said on Tuesday.

The U.S. central bank, which last week launched a potentially massive policy-easing effort with no set end date, will closely watch the ailing labor market for meaningful signs of improvement, the Fed policymakers said.

In response to lackluster economic growth that has not been enough to drive the unemployment rate down from levels above 8 percent, the Fed headed deeper into uncharted policy territory with a third round of quantitative easing, or QE3.

William Dudley, president of the New York Federal Reserve Bank on Tuesday said the economy needed a "nudge in the right direction," while Charles Evans, head of the Chicago Fed, predicted the central bank will keep buying assets at its current $85 billion-per-month pace into the new year.

"If the economy is weaker, we'll do more" asset purchases, said Dudley, a close ally of Fed Chairman Ben Bernanke and a key barometer of the thinking inside the central bank. "If the economy is stronger and we see a substantial improvement in the outlook for the labor market sooner, we'll end up doing less."

Dudley, addressing the Morris County Chamber of Commerce in New Jersey, added: "If you're trying to get a car moving that is stuck in the mud, you don't stop pushing the moment the wheels start turning - you keep pushing until the car is rolling and is clearly free."

Last week the Fed said it plans to buy $40 billion every month in mortgage-backed securities until the labor market outlook improves substantially.

The purchases come on top of an existing stimulus program in which the central bank buys about $45 billion a month in long-term Treasuries while selling the same amount of short-term Treasuries. That program, dubbed Operation Twist and designed to drive down long-term borrowing rates such as mortgages, runs through the end of 2012.

Evans, who has long advocated such aggressive action by the Fed, said he would be surprised if there is enough evidence by year-end to halt Treasury purchases altogether.

"Under those conditions, I would expect we would continue with something like an $85 billion base of purchases....that's a benchmark to start from," he told reporters after a speech in Ann Arbor, Michigan.

The Fed in late 2008 slashed interest rates to near zero and has since bought $2.3 trillion in securities in an unprecedented drive to spur growth and revive the economy after the worst recession in decades. Yet the recovery, especially in jobs, has been slow, leading the central bank to say it expects to keep rates at rock bottom at least through mid 2015.

Wall Street economists have been trying to pinpoint exactly what conditions would constitute a "substantial" improvement in the outlook for the labor market, which the Fed last week suggested would halt the new money-printing program.

Addressing this question, Dudley said the Fed will watch all corners of the labor market including payroll growth, the number of Americans who have given up the hunt for work, the employment-to-population-ratio, and job-finding rates, as well as the broader measure of unemployment.

At year end, further purchases of Treasuries will depend on an assessment of costs and benefits and on labor improvement, said Dudley, who as head of the important New York regional Fed bank has a permanent vote on Fed policy.

Ultimately, the Fed is looking for a stronger recovery alongside stable prices, said Dudley, who as head of the key New York Fed bank has a permanent vote on monetary policy. "When that finally materializes, I'll view it as consistent with the result we are trying to achieve, and not a reason to pull back our policies prematurely," he added.

INTERNAL POLICY RIFTS

Fed policymakers broadly agree that U.S. unemployment is much too high; most also agree that inflation, which has hovered near the Fed's 2 percent target, is well under control. But there continue to be deep rifts within the central bank over the best policy response.

Dallas Fed President Richard Fisher, a forceful opponent of further easing, said he would have dissented last week if he had a vote on the bank's policy-setting Federal Open Market Committee this year.

"I would argue that it is less impactful right now because you have other things inhibiting businesses from making decisions on capex and employment," Fisher told CNBC. "I don't think this program will have much efficacy."

Fisher has long said that uncertainty over tax policy and regulation are keeping firms on hold, inhibiting hiring and dampening growth.

Alluding to the looming "fiscal cliff" - the roughly $500 billion in expiring tax cuts and government spending reductions scheduled to take hold at the start of next year - Fisher said monetary policy alone could not solve the economy's ills.

"We cannot do it alone, and if it gets to the point where we are expected to do it alone ... we get into a very difficult spot from which we cannot exit," he said.

"So it's up to your viewers and everybody else, they own the Congress, they vote for them. Get your act together, Congress."

Only one of the 12 Fed voting policymakers dissented against QE3, which came very close to a plan Evans has advocated for the past year: a vow to keep rates low until unemployment drops below 7 percent or inflation threatens to top 3 percent, and to buy bonds if progress on jobs is not fast enough.

"I am optimistic that we can achieve better outcomes through more monetary policy accommodation," Evans told a group of business people at a breakfast sponsored by the Bank of Ann Arbor. "This is the time to act," he said, adding asset purchases could begin to taper in 2014 if the labor market improves as he expects.

Evans cast the debate over monetary policy as one between optimists who believe further easing can deliver a stronger economy, and pessimists who say it will only spark inflation. Pessimists have warned for years about higher inflation, only to have their predictions fall short, he said.

Risks abound that could send the U.S. economy back into recession, he said, citing a potential global slowdown, spillover from Europe's sovereign debt crisis and the looming "fiscal cliff.".

"We cannot be complacent and assume that the economy is not being damaged if no action is taken," Evans said.

(Additional reporting by Lucia Mutikani in Washington; Editing by Chizu Nomiyama and Leslie Adler)