* Weekly jobless claims drop 12,000
* Four-week average of claims fall 500
* Mid-Atlantic factory slows slightly in August
WASHINGTON, Aug 17 (Reuters) - The number of Americans filing for unemployment benefits fell to near a six-month low last week, pointing to a further tightening in the labor market that could encourage the Federal Reserve to lay out a plan to start unwinding its massive bond portfolio.
Labor market strength was corroborated by other data on Thursday showing manufacturers in the mid-Atlantic region sharply increased hours for workers in August amid a jump in new orders and unfilled orders.
Initial claims for state unemployment benefits dropped 12,000 to a seasonally adjusted 232,000 for the week ended Aug. 12, the Labor Department said.
That was the lowest level since the week ended Feb. 25 when claims fell to 227,000, which was the best reading since March 1973. Data for the prior week was unrevised.
It was the 128th week that claims remained below 300,000, a threshold associated with a robust labor market. That is the longest such stretch since 1970, when the labor market was smaller. The unemployment rate is 4.3 percent.
Economists polled by Reuters had forecast claims dropping to 240,000 in the latest week. A Labor Department official said there were no special factors influencing the claims data and that no states had been estimated.
The four-week moving average of claims, considered a better measure of labor market trends as it irons out week-to-week volatility, fell 500 to 240,500 last week.
Prices of U.S. Treasuries were trading lower as were U.S. stock index futures. The dollar was stronger against a basket of currencies.
LABOR MARKET STRENGTH
Last week's claims data covered the survey week for the August nonfarm payrolls. The four-week average of claims fell 3,500 between the July and August survey periods, suggesting another month of solid job growth.
Payrolls increased by 209,000 jobs in July. The economy has added 1.29 million jobs this year and the unemployment rate has fallen five-tenths of a percentage point.
Labor market tightness has, however, failed to generate strong wage growth, contributing to inflation consistently below the Fed's 2 percent target.
Minutes of the U.S. central bank's July 25-26 policy meeting showed policymakers appeared increasingly cautious about weak inflation, with some urging against further interest rate increases.
But labor market strength is probably sufficient for the Fed to outline a proposal to begin offloading its $4.2 trillion portfolio of Treasury bonds and mortgage-backed securities at its next policy meeting in September.
Most economists expect another rate hike in December. The Fed has increased borrowing costs twice this year.
In a second report on Thursday, the Philadelphia Fed said its index of manufacturing in the mid-Atlantic region slipped to 18.9 this month from 19.5 in July.
Despite the modest pullback, manufacturers reported increased demand for their products. The survey's measure of new orders surged to 20.4 from 2.1 in July. Firms also reported that shipments continued to rise.
As a result, workers put in more hours. The average workweek index increased to 18.8 in August from 3.8 in the prior month.
A third report from the Fed showed manufacturing output fell 0.1 percent in July as the production of motor vehicles and parts tumbled 3.6 percent. (Reporting by Lucia Mutikani; Editing by Paul Simao)