* Copper falls to 1-mth low on negative US credit outlook
* China raises banks' required reserves, 4th time this yr
* Macquarie sees 550,000T copper in China bonded sheds
* Coming up: U.S. housing data on Tuesday (Recasts, adds New York dateline/byline, updates with New York closing copper price, adds details and analyst comments)
By Chris Kelly and Rebekah Curtis
NEW YORK/LONDON, April 18 (Reuters) - Copper fell for the sixth straight session on Monday to a one-month low after a credit agency's negative credit outlook for the United States added to concerns about demand prospects.
Mounting concerns about Europe's debt problems added to that uncertainty, as unease over Greek debt and Portugal's pending bailout by the European Union exacerbated anxiety over how European policymakers will handle the region's festering debt and bank problems.
Ratings agency Standard & Poor's revised its credit outlook on the United States to negative from stable, citing a "material risk" that policymakers may not reach agreement on a plan to trim the country's large budget deficit.
"It just kind of pulled the rug out from under everything," said Phillip Streible, senior market strategist with Lind Waldock in Chicago.
Copper was already under some mild pressure overnight after China, the world's largest consumer, moved to raise banks' reserve requirements for the fourth time this year, extending its fight against excessive liquidity and stubbornly high inflation.
The Reuters-Jefferies CRB index lost more than 1 percent Monday in its sharpest one-day loss in about a week.
London Metal Exchange (LME) three-month copper hit a session trough at $9,207 a tonne, its lowest since March 17, before ending down $180 at $9,225.
COMEX May copper dropped 5.95 cents, or 1.4 percent, to settle at $4.1980 per lb, near the bottom of its $4.1720 to $4.2880 session range.
Trading volumes picked up in the sell-off, with a little more than 61,600 lots traded by 2:05 p.m. EDT (1805 GMT), about 35 percent above the 30-day average, according to preliminary Thomson Reuters data.
The S&P's worrisome outlook for long-term U.S. debt added another head wind to a market still coping with sluggish physical demand from China's monetary tightening campaign and economic recovery uncertainties tied to Japan's earthquake and subsequent tsunami last month.
"This provided a bit of a negative backdrop because obviously the U.S. is still a significant consumer (of copper) and it is a driver of financial sentiment," said Credit Agricole analyst Robin Bhar.
Bart Melek, vice president and director of commodities, rates research & strategy with TD Bank Financial Group, agreed.
"Eventually, this kind of stuff has an impact on the real economy, and it's the real economy that drives everything."
The United States is the second-largest copper consumer, with North America accounting for some 13 percent of global refined consumption. But China eclipses that by far, accounting for 38 percent.
PHYSICAL INTEREST
The amount of copper held to back the physical exchange-traded product of ETF Securities rose more than 250 tonnes to 3,251.550 tonnes on Monday, the company's website showed on Monday.
But this was more than overshadowed by the 1,350-tonne build in LME copper warehouses, bringing total inventory levels to 451,775 tonnes, their highest since June 2010.
The majority of these inflows went into warehouses in Asia, supporting investor fears of a sustained softness in Chinese demand.
Also, the potential that stockpiles of copper held in bonded warehouses in Shanghai could be unlocked and released to the market created additional concerns.
"We believe there is around 550,000 tonnes of copper sitting in bonded warehouses as of mid-April 2011, virtually unchanged from mid-March this year, but significantly higher than the level at end 2010 of estimated 400,000 and averaging 200,000-300,000 in the past," said Macquarie bank in a note.
"Most of the material ... is available for sale, so once the arbitrage starts to move in favour of import, the bonded stock could easily become customs cleared and sold onto the domestic physical market," it added. Metal Prices at 1809 GMT COMEX copper in cents/lb, LME prices in $/T and SHFE prices in yuan/T Metal Last Change Pct Move End 2010 Ytd Pct
move COMEX Cu 420.20 -5.55 -1.30 444.70 -5.51 LME Alum 2673.00 -17.00 -0.63 2470.00 8.22 LME Cu 9220.00 -185.00 -1.97 9600.00 -3.96 LME Lead 2528.00 -123.00 -4.64 2550.00 -0.86 LME Nickel 25505.00 -650.00 -2.49 24750.00 3.05 LME Tin 32350.00 -750.00 -2.27 26900.00 20.26 LME Zinc 2325.00 -73.00 -3.04 2454.00 -5.26 SHFE Alu 16870.00 60.00 +0.36 16840.00 0.18 SHFE Cu* 70960.00 240.00 +0.34 71850.00 -1.24 SHFE Zin 18425.00 5.00 +0.03 19475.00 -5.39 ** Benchmark month for COMEX copper * 3rd contract month for SHFE AL, CU and ZN SHFE ZN began trading on 26/3/07 (Additional reporting by Silvia Antonioli and Melanie Burton in London; editing by Jason Neely and Jim Marshall)