* Central bank could adjust interest rate guidance in Sept.

* Economic data better, anxiety among policymakers growing

* Yellen nods to less dovish views on labor market

By Jonathan Spicer, Michael Flaherty and Howard Schneider

JACKSON HOLE, Wyo., Aug 24 (Reuters) - Pressure is building within the Federal Reserve for officials to move as early as next month to more clearly acknowledge improvements in the U.S. economy and lay the groundwork for the central bank's first interest rate hike in nearly a decade.

According to some U.S. central bankers and their close advisers, signs of economic resilience and growing anxiety about the risks of holding rates too low for too long have set the stage for an intense debate over rewriting their policy statement.

It is uncertain whether officials will use their upcoming meeting on Sept. 16 and 17 to scrap key parts of the language they have been using to keep rate-hike expectations at bay, but if they do not, October looks like a good bet.

"Some shift of language is on the table, and should be on the table in the coming meetings," Atlanta Federal Reserve Bank President Dennis Lockhart, a policy centrist, said in an interview. While a handful of officials have argued for prompt changes, Lockhart said he thinks September "is still early."

Adding, dropping or adjusting even a few words in the Fed's post-meeting statement is a potentially treacherous task. A miscommunication by the world's most powerful central bank could shock financial markets globally and, in a worst case, reverse the economic recovery it seeks to foster.

At issue is a 5-month old pledge from the Fed to keep benchmark rates near zero for a "considerable time" after it shelves an asset-purchase program in October.

Another line that has drawn internal objections is the month-old statement that "significant" slack remains in the labor market, a suggestion that not even strong job growth and a further drop in unemployment will prompt a tightening of policy any time soon.

"The language puts us in a box that I think is not a good box to be in," Philadelphia Fed President Charles Plosser told Reuters on the sidelines of the central bank's annual Jackson Hole conference.

Plosser dissented against the "considerable time" line at the Fed's last meeting in late July. Like fellow hawks at the central bank, he said he prefers "very simple, data-dependent" guidance that avoids timelines or calendar dates.

WARY EYE ON GLOBAL GROWTH

Aside from an unpredictable market reaction, Fed Chair Janet Yellen will have to contend with a potentially uncooperative world economy that could upend U.S. economic progress.