PARIS — The economy of the euro zone expanded this month at its fastest rate in nearly three years, according to a private sector survey released on Wednesday, with accelerated output from Germany leading the bloc.

Markit’s composite index of economic activity, based on a survey of purchasing managers across the 18 nations that use the euro, rose to 54 in April from 53.1 in March — the strongest growth since May 2011. It was the 10th consecutive monthly increase.

A reading of 50 or higher signals growth, while a level below 50 suggests contraction. Economists had been expecting an index reading of about 52.9.

Subindexes showing activity in the manufacturing and service sectors also rose. Analysts watch the indexes carefully, considering them to be among the best “real-time” guides to the euro zone economy.