* Asia shares follow Wall St down, Nikkei skids over 2 pct
* Shanghai copper futures lowest since 2009, gold hits 4-1/2 mth high
* Reports China central bank ready to ease policy if needed
* Yen edges higher as Australian dollar loses ground
By Wayne Cole
SYDNEY, March 12 (Reuters) - Asian stocks fell on Wednesday as economic uncertainty in China and the United States combined with political tensions in Ukraine to keep investors cautious and commodities under a dark cloud.
Copper grabbed the headlines as Shanghai futures fell almost 4 percent to their lowest since 2009 amid concerns about the potential unravelling of loan deals in China where the industrial metal has been used as collateral.
Japan's Nikkei retreated 2.2 percent, continuing the see-saw pattern of the last couple of months, while Australian stocks shed 0.6 percent. MSCI's broadest index of Asia-Pacific shares outside Japan fell 1.1 percent.
That mirrored a lacklustre performance on Wall Street, where soft data left investors no wiser on whether the U.S. economy's troubles were merely weather-related or something more worrisome.
The Dow ended on Tuesday down 0.41 percent, while the S&P 500 lost 0.51 percent.
In Asia, investors were hoping Chinese data on industrial output, retail sales and urban investment on Thursday might offer some clarity on where the economy is heading.
One potential positive was a Reuters report that China's central bank is prepared to loosen monetary policy if economic growth slows further by cutting the amount of cash that banks must keep as reserves.
Citing sources involved in internal policy discussions, the report said an easing would be triggered if growth slips below 7.5 percent, and would come on top of money market operations and currency intervention via state banks that traders say has already loosened monetary conditions overall.
In the meantime, caution was the watchword as concerns about the state of Chinese demand continued to pressure industrial commodities, particularly copper and iron ore.
Both metals have been used in China as collateral for loans, leaving traders and steel mills vulnerable to a credit squeeze.
"Copper prices continue to slide on concerns that the cyclical metal may face tough times as China slows and rebalances away from investment into consumption," said analysts from Barclays in a note.
At least one U.S. scrap copper trader has suffered "large" losses after a buyer in China defaulted on a deal in the past week, one of the first signs that sinking prices and tightening credit are taking a toll on the physical market.
"Global risk sentiment has held up well, however, as so far the market does not seem to identify these risks as systemic," noted Barclays.
The relative calm was clear in the U.S. Treasury market, where yields on 10-year notes have hovered around 2.77 percent for the past couple of sessions.
Still, the slide in prices for copper and iron ore undermined Australia's dollar as the country is a major exporter of both metals. The currency was down at $0.8962 on Wednesday, having shed half a U.S. cent overnight.
It was one of the few movers among the major currencies, which have been trading in tight ranges recently. The dollar eased a touch on the yen to 102.97 as the Japanese currency benefited from its traditional status as a safe haven.
The euro edged lower to $1.3855 after officials from the European Central Bank reminded investors there was still scope to ease policy if needed.
Lurking in the background was the standoff in Ukraine. Ukraine's acting president announced the formation of a volunteer national guard, while ousted leader Viktor Yanukovich insisted he remained the legitimate leader.
One beneficiary of global uncertainty has been gold, which jumped to a 4-1/2 month high of $1,359.25 an ounce on Wednesday.
Oil prices extended their pullback with Brent crude off 26 cents to $108.29 a barrel, while U.S. oil lost 61 cents to $99.42.