By Neal Armstrong

LONDON, Sept 8 (Reuters) - The yen struck a fresh 15-yearhigh against the dollar and the Swiss franc hit an all-time peakversus the euro on Wednesday as a flare-up in worries over eurozone banks and sovereign debt led investors into safe havens.

Persistent buying by investors seeking a temporary refugein the yen helped push the greenback through a major optiontrigger below 83.50 yen, testing the Japanese authorities' painthreshold for strength in their currency.

"There's been an intensification in verbal rhetoric fromJapan but I don't think they will intervene here. The seriousintervention risk comes in below 80 yen," said Manuel Oliveri,currency strategist at UBS in Zurich.

Bank of Japan Governor Masaaki Shirakawa reiterated hisreluctance to return to quantitative easing although heindicated the central bank was weighing its options on how todeal with the economic impact of the yen's strength.

Finance Minister Yoshihiko Noda again warned he would takedecisive action if necessary, but some in the market thought theJapanese would struggle to make intervention effective.

"The growth and changing nature of fx since the Japaneselast intervened should dampen their hopes for now. The speed ofthe move will be the cue for the BOJ. My fresh target fordollar/yen is now 79.50 and for euro/yen 104.50," said a traderat a U.S. bank in London.

The dollar fell as far as 83.34 yen, down 0.5 percent on theday and its cheapest since 1995 when it struck an all-time lowaround 79.75. It later recovered towards 84.00 on buying by amajor European bank in thin liquidity.

The euro fell 0.5 percent to 105.80 yen at one point,threatening to revisit August's nine-year low just below 105.50,but recovered to 106.50 by 1007 GMT.

"The euro's weakness is worrying. If the euro becomesunstable, there could be more selling in euro/yen, which woulddrag dollar/yen lower too," a trader at a Japanese trading housesaid.

The euro was close to flat at $1.2700, having dived from$1.2870 on Tuesday and a three-week high of $1.2920 the daybefore.

Technical analysts saw the next support at $1.2605, the 50percent retracement of the euro's rally from May to August. Theeuro dipped under that level briefly last month but did notclose below.

SWISS FRANC IN DEMAND

The euro fell to a record low versus the Swiss franc of1.2766 francs after taking out the previous day's record low of1.2812 francs, and traders saw little chart support until1.2700.

"There's real safe-haven demand for the Swiss and the eurois suffering from a shift back on to issues in the euro zonebanking sector," said Oliveri at UBS.

A newspaper report on Tuesday reignited concerns aboutEuropean sovereign debt and banks' exposure.

A Portuguese bond auction on Wednesday showed investorsremained sensitive to peripheral debt risks as they demanded anaverage yield of close to 6 percent to hold 2021 Treasury Bonds,higher than 5.3 percent at the previous offering.

(Additional reporting by Hideyuki Sano, editing by NigelStephenson)