* MSCI Asia ex-Japan falls to 2013 low, Nikkei sheds as much as 2.4 pct
* BOJ inaction to calm JGBs, Fed stimulus concerns unnerve markets
* Commodities weighed by central bank policy speculation
* European shares likely to fall
By Chikako Mogi
TOKYO, June 12 (Reuters) - Asian shares hit fresh 2013 lows and Japanese stocks had another volatile session on Wednesday, extending a broad rout in global equities, as the lack of new steps from the Bank of Japan to quell tumult in the domestic bond market and lingering fears of a softening of U.S. stimulus unnerved investors.
European stock markets are likely to fall, with financial spreadbetters predicting London's FTSE 100, Paris's CAC-40 and Frankfurt's DAX will open down 0.5 percent. A 0.2 percent rise in U.S. stock futures, however, suggested a stable start on Wall Street.
The BOJ's move came amid mounting concern that central bank support for markets was turning more cautious, sparked by persistent speculation about the U.S. Federal Reserve toning down its strong stimulus commitment later in the year.
These concerns continued to buffet markets and led to a massive selloff in global equities and commodities overnight, although the dollar recouped some of Tuesday's sharp losses against the yen.
The dollar was trading up 0.9 percent at 96.89 yen after falling more than 2 percent to a low of 95.59 yen overnight. The dollar hit a two-month low of 94.975 on Friday, having just seen a 4-1/2-year peak of 103.74 yen last month.
The possibility of a shift in the Fed's current policy, even if such a move wasn't imminent, has rattled markets in recent weeks as the Fed's aggressive bond-buying plan has been a major driving force behind the recent rally in global risk assets.
"There appeared to be some heightened anxiety levels across Asian markets today as traders contemplate a world without additional economic stimulus," said Tim Waterer, senior trader at CMC Markets in Sydney.
MSCI's broadest index of Asia-Pacific shares outside Japan fell 0.4 percent to a 6-1/2-month low, extending losses into the sixth straight session, the longest losing streak since March last year.
Australian shares fell 0.9 percent to a five-month low while South Korean shares ended down 0.6 percent. Markets in China and Hong Kong are closed for holidays.
"Market reactions of late underscore the liquidity risk -- bond buying schemes are reducing liquidity in bond markets and overshooting prices," said Takeo Okuhara, fund manager at Daiwa SB Investments. "Markets have long been too complacent with central bank support."
Japan's Nikkei stock average closed down 0.2 percent in a volatile session which saw the benchmark tumble as much as 2.4 percent earlier. The Nikkei is down about 17 percent from a 5-1/2-year high scaled last month.
The BOJ on Tuesday held off from taking additional steps to curb bond market volatility, arguing that bond markets had stabilised. BOJ Governor Haruhiko Kuroda said that the central bank will consider fresh measures if borrowing costs spike again in the future.
The lack of new action, following an unprecedented bond-buying programme launched in April, disappointed some investors who were expecting the central bank to extend the maximum duration of cheap fixed-rate funds as a way to reduce volatility in the bond market.
GLOBAL GROWTH, STIMULUS WORRIES MOUNT
Markets have been on edge even before the latest BOJ decision rippled through asset markets. Volatility in global financial markets had heightened in recent weeks on the Fed stimulus jitters, worries over slowing growth in China, a deep slump in Europe, and shaky Japanese bonds and stocks which threatened to undercut the BOJ's stimulus moves.
Sentiment towards Japan has also been hit recently as investors began to wind down their excessive expectations for Prime Minister Shinzo Abe's pro-growth policies, suspecting that Abe might be holding off from announcing harsher structural reforms needed to invigorate Japan until after an election in July.
Since mid-November when expectations began building for Abe, who became prime minister in December, to pursue bold reflationary steps to pull Japan out of deep deflation, the dollar rose 30 percent against the yen and the Nikkei surged almost 80 percent to their respective peaks in May.
"Markets have yet to find a new focus that replaces the trading betting on Abe, which appears to be closing out. The next focus is not Japan, maybe the Fed, but in the absence of clear fresh factors, each asset may be undergoing position adjustments under the current volatile conditions," said Hiroshi Maeba, head of FX trading Japan for UBS in Tokyo.
The prospect of the Fed withdrawing some of its funds from markets pressured emerging currencies, prompting Indonesia's central bank to say it was ready to ensure large supplies of dollars to the market to support the weakening rupiah.
The idea of less bond buying has also been behind a recent spike in U.S. Treasury yields to their highest in more than a year.
Commodities were also weaker, with London copper easing 0.2 percent to $7,047.75 a tonne, hovering near a six-week low.
U.S. crude futures slipped 0.9 percent to $94.54 a barrel and Brent fell 0.6 percent to $102.30.
Spot gold extended declines into a second session, easing 0.3 percent to $1,374.50 an ounce, as investors weighed the risk of the Fed curbing its stimulus programme.