* Dollar index hits 3-mth high; euro/dollar tumbles
* Euro falls through $1.4500 barrier, hits stops beneath
* S&P downgrade of Greece dents euro
* More upbeat Fed economic view helps dollar
(Updates prices; changes byline, dateline; previous TOKYO)
By Jessica Mortimer
LONDON, Dec 17 (Reuters) - The dollar surged to a three-month high against a currency basket on Thursday, buoyed by an upbeat tone from the Federal Reserve, while the euro tumbled on fresh concerns about Greece's waning fiscal health.
In thin pre-holiday liquidity, the euro spun to its lowest against the dollar since early September, picking up pace after breaking strong support at $1.4500 and hitting stop-loss sell orders to take it below the $1.4400 mark.
Concerns about fiscal troubles in some euro zone countries gathered pace as Greece suffered the second downgrade of its credit rating in a week on Wednesday.
Standard & Poor's cut Greece's rating by one notch, to BBB-plus from A-minus, saying austerity steps announced by Prime Minister George Papandreou this week were unlikely to produce a "sustainable" reduction in the public debt burden.
A statement by the Federal Reserve as it left interest rates steady also supported the dollar. It voiced growing optimism on the U.S. economy as job losses slow, but repeated a vow to keep interest rates unusually low for "an extended period".
"The problem for the euro is the mix of the FOMC statement and the very strong concerns over Greece after the S&P downgrade. All the euro crosses have suffered," said Roberto Mialich, FX strategist at Unicredit in Milan.
"The break in euro/dollar below $1.45 leaves $1.43 and $1.42 as the next target, with a potential rebound unlikely in the near term," he said.
The euro fell more than 1 percent to $1.4369 on trading platform EBS, its lowest since early September. By 0858 GMT it was trading at $1.4388, down 1 percent.
The single currency also dropped more than 1 percent against the yen and hit a one-month low versus sterling
Sentiment towards the euro has been dented by fiscal health worries in some countries, angst over European banks and a view that the U.S. economy may be recovering faster than the euro zone.
A further negative came as S&P on Wednesday put about 1.46 trillion euros worth of covered bonds on credit watch negative or developing.
The European Central Bank has bought about 27 billion euros of covered bonds since the summer to help revive the market after the credit crisis.
DOLLAR GAINS
The dollar index, a gauge of its performance against six major currencies, rose as far as 77.609, its highest since early September, giving investors more confidence that the U.S. currency has broken out of a downtrend that began in March.
It last traded up 0.7 percent at 77.531.
The dollar has been dogged this year by the prospect of interest rates staying close to zero, but it broke out of its downtrend earlier this month as U.S. economic data improved.
On Wednesday, its 14-day moving average crossed up through the 30-day moving average, viewed by chartists as a bullish signal.
Investors are increasingly confident the dollar is reacting positively to strong U.S. data, a move away from its previous trend of rising in times of heightened investor risk aversion.
The higher yielding Australian dollar, which has been a popular buy against the dollar this year, tracked the euro down, falling as far as $0.8872, its weakest since early October. It stood at $0.8908 by 0858 GMT, down 1.1 percent.
The Aussie has been under pressure since Reserve Bank of Australia Deputy Governor Ric Battellino surprised markets on Wednesday by signalling interest rates might not rise as much as investors were expecting.
"Dollar buying interest is especially strong against the euro and the Aussie. This move suggests to me that the dollar looks like it has started another bull run before year-end," said a trader for a Japanese trust bank in Tokyo.
(Additional reporting by Satomi Noguchi in Tokyo)