(Bloomberg) -- BlackRock, the biggest provider of ETFs, is ramping up its fight with Vanguard for U.S. retail investors by doubling the number of funds in its low-fee “core” series.

BlackRock’s iShares unit cut fees on six existing funds and opened four new products, the New York-based company said today in a statement. BlackRock introduced its core lineup in October 2012 and rolled out a similar group of funds in Europe last week.

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