Assets held in U.S. exchange-traded funds declined slightly in May, according to monthly statistics collected by State Street Global Advisors.

As of May 31, 885 exchange-traded funds in the United States had $748 billion in assets, down 5.5% from a month earlier.

Exchange-traded funds were hampered by a decline in global equity markets in May amid concerns over the European debt crisis. The Standard & Poor's 500 Composite Index declined 8% while the MSCI EAFE Index fell 11.4%. U.S> bonds rose as the Barclays U.S. Treasury Index up 1.7% and the Barclays U.S. Aggregate Index up 0.8%.

As for ETFs, declines in the size and international categories accounted for nearly three quarters of the total drop in ETF assets under management, according to State Street's data, which was released last week. Only the fixed income, commodity and inverse/leveraged sectors had an increase in assets.

Large-cap ETFs declined $12.1 billion, followed by mid-cap ETFs, which were down $2.6 billion.

The three largest managers in the U.S.-based ETF market, BlackRock, State Street and Vanguard, collectively accounted for approximately 84% of the assets in the sector, though their collective marketshare declined 0.2% through the first five months of this year.