By Claire Milhench

LONDON (Reuters) - Global investors raised holdings of Japanese bonds to four-year highs in September and bought more euro zone debt, shrugging off a mid-month market sell-off and expressing faith in central banks' commitment to their asset-buying programs.

The latest Reuters monthly poll of 45 fund managers and chief investment officers in the United States, Europe, Britain and Japan, showed Japanese bond holdings rising to 15.9 percent of global portfolios, the highest level since June 2012.

The survey, conducted between Sept 15-28, also showed investors raising euro zone holdings to 27.9 percent of debt portfolios, the highest level since February 2016.

The moves came despite a market rout early in the month, when yields on 30-year Japanese bonds hit their highest in six months. Ten-year U.S. Treasury yields jumped to three-month highs and German 10-year yields inched above zero for the first time in three months.

The yield curve shake out was driven by worries that the post-2008 era of super-charged central bank stimulus was coming to an end.

These fears were somewhat allayed by the Bank of Japan's comments after its Sept 21-22 meeting, in which the central bank said it would allow inflation to overshoot its 2 percent target before tapering asset purchases.

It also committed to keeping 10-year bond yields around zero percent in an attempt to steepen its yield curve.

"Global central banks, first and foremost the Bank of Japan, made it clear that they are still committed to their purchase programs and are willing to support the market for longer," said Jan Bopp, an asset allocation strategist at Bank J Safra Sarasin.

This was important, he said, as investors had started to worry that central banks would call time on quantitative easing.

Such fears had been encouraged by the European Central Bank's failure to announce new stimulus measures at its Sept. 8 meeting but after a brief sell-off, bond prices recovered.

As a result, some 72 percent of survey respondents who answered a specific question about the bond rout, said they did not see a further sell off in G3 bonds this year.

"The fact remains that whilst the central banks continue to be the biggest buyers in the markets through monthly bond-buying programs, it is likely that bond prices will remain inflated and yields depressed," said Peter Lowman, chief investment officer at UK-based wealth manager Investment Quorum.

Raphael Gallardo, a strategist at Natixis Asset Management said that because global economic growth was not expected to accelerate, central banks would not allow a further bond sell off to endanger the fragile recovery.

U.S. bond exposure fell, however, to 36.3 percent, the lowest since May.

CAUTIOUS ON EQUITIES

There was little change in investors' overall bond and equity allocations, with the latter held at 44 percent, and bonds steady at 41.3 percent, the highest in at least five years.

Equity markets continue to face headwinds, investors said.

"Volatility tends to increase as we head into autumn," said Trevor Greetham, head of multi-asset at Royal London Asset Management.

"Near-term risk factors include the U.S. presidential election, the possibility of a weak patch in economic data and political tensions in Europe."

But the U.S. Federal Reserve held off raising rates this month and its projections of gradual policy tightening helped spur the Nasdaq <.IXIC> index to record highs.

The share of U.S. stocks in global equity portfolios rose to 41 percent, up from 38.5 percent in August -- the highest in just over a year.

But Euro zone and Japanese equity allocations fell to 17.7 percent and 17 percent respectively.

European investors in particular are fretting about the fate of giant lender Deutsche Bank <DBKGn.DE>, whose shares have sunk to record lows after being hit with a U.S. fine of up to $14 billion. Its woes have hurt European shares this month, especially German equities, which are down 3 percent <.DAX>.

Doubts also lingered about the effectiveness of central banks' stimulus programs, with Matteo Germano, global head of multi-asset investments at Pioneer Investments, questioning how far central banks could go to cushion shocks and address structural weaknesses.

"Whether there are bullets left to face a potential crisis or slowdown is open to question. In this scenario we prefer a cautious approach to equities," he said.

Some of the cash taken from European shares found its way into emerging markets, with holdings of emerging European stocks jumping to 3 percent, the highest in at least five years.

Boris Willems, a strategist at UBS Asset Management, said that although emerging equities had been cheap for some time, they had lacked the catalyst to overcome structural issues such as excess industrial capacity and sizeable external debt.

He recently raised exposure however, saying, "There is now scope for a positive earnings surprise in the short to medium- term."

(Additional reporting by Maria Pia Quaglia Regondi; Editing by Catherine Evans)