BEIJING—China’s economic expansion languished to its slowest pace in nearly three decades last year, as a bruising trade fight with the U.S. exacerbated weakness in the world’s second-largest economy.

The 6.6% growth rate for 2018 reported Monday is the slowest annual pace China has recorded since 1990. The economic slowdown, which has been sharper than Beijing expected, deepened in the last months of 2018, with fourth quarter growth rising 6.4% from a year earlier.

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