BEIJING—China’s services sector suffered an unexpectedly severe blow in August as a wave of coronavirus infections sparked new lockdowns across the country, sending an official gauge of nonmanufacturing activity into contractionary territory for the first time since the country’s pandemic recovery began more than a year ago.

China’s official nonmanufacturing purchasing managers index, which tracks activity in the construction and services sectors, plummeted to 47.5 in August, from 53.3 the prior month, according to data released Tuesday by the National Bureau of Statistics, breaking through the 50 mark that separates expansion from contraction.