Economists had expected China's trade surplus to have narrowed to $37 billion in December from November's $40.21 billion.
Strong global demand saw China's foreign trade activities increase considerably in 2017, the General Administration of Customs reported Friday. That's the smallest monthly value since at least January 2014.
The trade deficit with China is the U.S.'s largest with any trading partner and 2017's is the biggest with China in the almost five decades for which records exist. But bilateral ties, which date back to the Korean War, have weakened since North Korea began testing nuclear weapons in 2006 and Beijing has since joined the global community in supporting United Nations sanctions.
Iron ore futures on the Dalian Commodity Exchange dropped 2.1 percent to 544 yuan ($84.28) a tonne on Friday.
A massive government push to heat millions of homes and power thousands of factories with natural gas in northern China has led to demand for the fuel outpacing supply, while delivery infrastructure has found it hard to cope.
The European Union, US and ASEAN remain China's top three trade partners, representing almost 42 percent of the total volume, while trade with Russia, Poland and Kazakhstan saw rapid growth of between 20 and 40 percent due to the new Silk Road scheme.
Chinese exports to North Korea was also down 23.4 percent to $260 million, Yonhap reported.
In December alone, China imported 7.9 million tonnes of natural gas with total value of 19.8 billion yuan (about 3 billion USA dollars).
In December, China's exports jumped 10.9% from a year ago to $231.8 billion, down from a 12.3% pace in the previous month.
In November, China stopped exporting oil products to North Korea, after the U.N. Security Council that month imposed new caps on trade with North Korea, including limiting oil product shipments.
China's chronically high trade surplus with the US hit a record level in 2017, adding fuel to Trump administration criticisms about Chinese trade practices just as it weighs a range of penalties and other actions to curb the imbalance.
Customs is expected to release dollar-denominated trade data later on Friday.