* Dollar index at 3-mth high; euro, stg, Swiss franc tumble
* Euro falls 2 cents vs dollar to $1.4330; Greece woes weigh
* More upbeat Fed economic view helps dollar
(Updates prices, adds quotes)
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 well below $1.4400.
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. [ID:nLDE5BG0AO]
Dollar buying intensified on Thursday as sterling fell to a two-month low against the greenback after data showing UK retail sales unexpectedly fell in November, and the Swiss franc and Australian dollar hit three- and two-month lows respectively.
The dollar index <.DXY>, a gauge of its performance against six major currencies, rose as far as 77.823, 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 0.9 percent at 77.720.
"The euro is suffering as a result of problems and debt issues relating to periphery euro zone countries which could pose a significant strain for the region, while the pound is under pressure due to a lacklustre economy," said Steve Barrow, currency strategist at Standard Bank.
He said the euro could fall to $1.42 or $1.41 in the near term.
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.
The euro fell around two U.S. cents from late New York trade to $1.4330, according to Reuters data, its weakest since early September. By 1109 GMT it was trading at $1.4355 <EUR=>, down 1.3 percent. It also dropped more than 1 percent against the yen <EURJPY=R>.
Sentiment on the euro has been hit 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.
Sterling <GBP=D4> hit a two-month low against the dollar of $1.6080, while the Swiss franc <CHF=> hit a three-month low of 1.0507 francs per dollar.
A statement by the Federal Reserve as it left interest rates steady 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.
As year end approaches, investors have been cutting back short dollar positions. Data last week showed the value of the dollar's net short position fell to around $11.78 billion in the week ending Dec. 8, from 21.8 billion the previous week [ID:nN11579430]
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 -- a move chartists saw as bullish.
The higher yielding Australian dollar <AUD=D4>, which has been a popular buy against the dollar this year, fell as far as $0.8872, its weakest since early October.
The dollar was unmoved by comments from Chinese central bank deputy governor Zhu Min. MNSI quoted him as saying dollar depreciation would continue and that it was getting harder for foreign governments to buy U.S. Treasuries as the U.S. current account shrank.
(Reporting by Jessica Mortimer)