Risks to global expansion have recently increased due to tensions in the U.S. subprime mortgage market and there are fears of growing spillovers to other segments, the European Central Bank said on Thursday.

"Risks also relate to global imbalances, protectionist pressures and the possibility of further increases in oil and commodity prices," the bank said in its monthly bulletin for September.

"While the global repercussions of the U.S. economic slowdown have so far been limited, it remains to be seen whether the recent financial market turmoil will lead to a lasting reappraisal of global financial market risks and a loss in confidence with possible implications for the real economy," the bulletin said.

Global economic activity remained robust, however, supported mainly by buoyant activity in emerging economies, the bank said.

"In particular, the larger countries in Asia continue to record strong growth rates, and the region remains the main engine of global growth," it added.