* ECB expected to intervene to stem euro zone crisis * Benchmark yields bumping against support at 1.87 pct * Fed sells $7.8 bln of shorter-dated Treasuries as part of Twist By Chris Reese NEW YORK, Aug 21 (Reuters) - U.S. Treasury debt prices fell on Tuesday as investors turned to riskier assets due to expectations the European Central Bank will eventually intervene to ease the euro zone debt crisis, although lack of detail about any plans limited losses. Willingness to take on more risk had investors bailing on Treasuries, while U.S. stocks touched a four-year high. Expectations of more ECB action were sparked by a weekend report in German magazine Der Spiegel saying the ECB was considering setting yield thresholds for any purchases of struggling euro zone country's bonds. However, the ECB tried to quash speculation on Monday that it would target specific interest rate thresholds as part of any bond-buying program. On Tuesday, an ECB spokeswoman referred back to Monday's statement, which had said it was misleading to report on policy decisions that had not been taken. Uncertainty about the ECB plans is high. Investors are also concerned the ECB's condition that troubled countries need to ask for help from the euro zone's rescue funds before getting assistance from the central bank may mean that the Spanish crisis could get worse before it gets better. Still, some optimism over eventual ECB action had investors looking to take on riskier assets and sell lower-risk U.S. government debt. Ten-year Treasury notes on Tuesday were trading 13/32 lower in price to yield 1.85 percent, up from 1.81 percent late Monday. "The market has moved to the belief that (the ECB) is going to do whatever it takes," said William Larkin, fixed income portfolio manager at Cabot Money Management in Salem, Massachusetts. He added fears over the eventual outcome of Europe's debt crisis, along with worries over the pace of global growth, had pushed Treasury debt prices to levels that many investors considered to be overly expensive. Benchmark yields have generally been rising since hitting a record low of 1.38 percent in late July. Losses were also limited on Tuesday by technical factors, with the 200-day moving average at 1.87 percent capping this month's rising trend in 10-year yields. Investors are speculating on whether the Federal Reserve will step in with another round of economic stimulus, and will parse through the minutes of the central bank's last policy meeting, set for release on Wednesday, for any clues as to whether more quantitative easing is on the way. The Fed sold $7.8 billion of shorter-dated Treasuries on Tuesday as part of its current stimulus program, which has been dubbed "Operation Twist." The central bank is selling shorter-dated U.S. government debt and buying longer-dated Treasuries in an effort to lower longer-term borrowing costs like those on mortgages. Thirty-year Treasury bonds were trading 30/32 lower in price to yield 2.97 percent, up from 2.92 percent late Monday.