* Dollar/yen drops to 15-year low at 83.34 yen JPY=
* Euro extends all-time low versus Swiss franc EURCHF=
* Market frets over euro zone banks, sovereign debt
* Portugal bond auction commands high average yield
(Adds quote, detail, updates prices)
By Neal Armstrong
LONDON, Sept 8 (Reuters) - The yen struck a fresh 15-year high against the dollar and the Swiss franc hit an all-time peak versus the euro on Wednesday as a flare-up in worries over euro zone banks and sovereign debt led investors into safe havens.
Persistent buying by investors seeking a temporary refuge in the yen helped push the greenback through a major option trigger below 83.50 yen, testing the Japanese authorities' pain threshold for strength in their currency.
"There's been an intensification in verbal rhetoric from Japan but I don't think they will intervene here. The serious intervention risk comes in below 80 yen," said Manuel Oliveri, currency strategist at UBS in Zurich.
Bank of Japan Governor Masaaki Shirakawa reiterated his reluctance to return to quantitative easing although he indicated the central bank was weighing its options on how to deal with the economic impact of the yen's strength.
Finance Minister Yoshihiko Noda again warned he would take decisive action if necessary, but some in the market thought the Japanese would struggle to make intervention effective.
"The growth and changing nature of fx since the Japanese last intervened should dampen their hopes for now. The speed of the move will be the cue for the BOJ. My fresh target for dollar/yen is now 79.50 and for euro/yen 104.50," said a trader at a U.S. bank in London.
The dollar fell as far as 83.34 yen JPY=, down 0.5 percent on the day and its cheapest since 1995 when it struck an all-time low around 79.75. It later recovered towards 84.00 on buying by a major European bank in thin liquidity.
The euro fell 0.5 percent to 105.80 yen EURJPY=R 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 becomes unstable, there could be more selling in euro/yen, which would drag dollar/yen lower too," a trader at a Japanese trading house said.
The euro was close to flat at $1.2700 EUR=, having dived from $1.2870 on Tuesday and a three-week high of $1.2920 the day before.
Technical analysts saw the next support at $1.2605, the 50 percent retracement of the euro's rally from May to August. The euro dipped under that level briefly last month but did not close below.
SWISS FRANC IN DEMAND
The euro fell to a record low versus the Swiss franc of 1.2766 francs after taking out the previous day's record low of 1.2812 francs, and traders saw little chart support until 1.2700.
"There's real safe-haven demand for the Swiss and the euro is suffering from a shift back on to issues in the euro zone banking sector," said Oliveri at UBS.
A newspaper report on Tuesday reignited concerns about European sovereign debt and banks' exposure.
A Portuguese bond auction on Wednesday showed investors remained sensitive to peripheral debt risks as they demanded an average yield of close to 6 percent to hold 2021 Treasury Bonds, higher than 5.3 percent at the previous offering. [IDnLIS002456]
(Additional reporting by Hideyuki Sano, editing by Nigel Stephenson)