In its quarterly review, the BIS said financial market volatility spiked higher in August on the back of geopolitical concerns and worries over economic growth, but quickly returned to "exceptional lows" across most asset classes.

"By fostering risk-taking and the search for yield, accommodative monetary policies thus continued to contribute to an environment of elevated asset price valuations and exceptionally subdued volatility," the BIS said.

The comments echoed the institution's warning earlier this year that rock-bottom interest rates had led to "worrying" signs of unsustainable growth in property and credit markets in some countries.

The U.S. Federal Reserve is on course to bring its bond-buying program to an end in October and is expected to begin raising interest rates next year.

But if the Fed is stepping back, the European Central Bank is stepping up. The ECB has cut key rates, will offer hundreds of billions of euros of liquidity to banks, and purchase hundreds of billions euros of assets to try and ward off deflation and revive flagging growth.

Anticipation of the ECB's largesse eclipsed concern over geopolitics and pushed credit spreads, bond yields and volatility back down again, the BIS said.

There were several references in the report to the "extraordinarily" and "exceptionally" low levels of volatility, suggesting the BIS feels markets may be getting too complacent and therefore vulnerable — and therefore ill-equipped to handle a shock.

Separately, the report also noted that overall international banking activity rose in the first quarter of this year, the first increase since 2011.

Cross-border claims of BIS reporting banks rose a "significant" $850 billion, not enough to offset previous quarterly declines but enough to slow the year-on-year decline to 2 percent from 4 percent.

The BIS serves central banks in their pursuit of monetary and financial stability, to foster international cooperation in those areas and to act as a bank for central banks. The organization was founded in 1930 and is based in Basel, Switzerland.