The ailing dollar fell on Wednesday as sliding U.S. Treasury yields and below-forecast U.S. data fueled expectations of further monetary easing.

Mounting speculation the Federal Reserve could embark on a second round of quantitative easing, which would be negative for the dollar, drove the greenback to a five-month low against the euro and a two-year trough against the Australian dollar.

"The market is jumping on QE expectations as it feels the U.S. data will force the Fed to do something, and I think that will be the case," said Manuel Oliveri, currency strategist at UBS in Zurich.

Analysts said the dollar could face further losses as a selling trend was taking hold, while the euro would continue rising after becoming resilient to economic and banking problems facing some countries on the eurozone periphery. "The safe bet is to keep selling the dollar, especially given reasonably supportive data from the euro," said Peter Frank, currency analyst at Societe Generale.

Data on Wednesday showed euro zone economic sentiment unexpectedly rose in September.

Frank said the euro had developed a thick skin to banking and economic problems faced by Ireland, which have flared in past weeks. Unless problems arose in a major country, such as Spain, the future of the euro zone was unlikely to be threatened.

At 1117 GMT, the dollar index was down 0.3 percent at 78.771, close to an earlier eight-month low of 78.616.

Dollar weakness helped push the Australian dollar to a two-year high of $0.9730 after a large option barrier at $0.9700 gave way.

The Swiss franc rose to 0.9735 francs per dollar, a 2-1/2-year high, according to Reuters data.

It later gave up those gains after data showed Switzerland's leading growth indicator, the KOF economic barometer, eased to 2.21 in September from 2.22 in August.

The KOF beat forecasts of 2.12, but some in the market had expected an even stronger reading.

The euro jumped to a five-month high of $1.3638 before pulling back to $1.3610.

Gains were capped after Standard and Poor's downgraded nationalized Anglo Irish Bank's lower tier 2 debt to CCC from B, but the euro traded 0.2 percent higher on the day.

Irish and Portuguese yield spreads hit euro lifetime highs against German bonds on Tuesday on concerns over those countries' fiscal deficits.

YEN INTERVENTION NERVES

The euro has risen about 11 percent against the dollar so far in the July-September quarter and is on track for its biggest quarterly percentage gain in about eight years, according to Reuters data.

"The primary reason for the euro's rise has been declining confidence in the dollar and covering of stale euro short positioning," said Stephen Gallo, head of market analysis at Schneider Foreign Exchange.

The dollar also looked vulnerable against the yen, hitting its lowest since Japan intervened to sell the yen two weeks ago to drive the dollar up from a 15-year low.

The dollar fell 0.4 percent against the yen to 83.50 yen on EBS, its lowest since Sept. 15, when Japan intervened.

Many in the market think Japan is likely to intervene again if the dollar threatens the 83 yen area as Japan's intervention began after it had hit a 15-year trough of 82.87 yen.