Export volumes declined 33 percent to 3.25 million tonnes, while the value of coal exports dropped 25 percent to $245.4 million in the first two months of the year, according to the National Statistics Office.

"The reason is obvious: the bottleneck at the border is causing exports to drop," said Mogi Badral Bontoi, chief executive and analyst for Cover Mongolia.

Bonto said the number of trucks passing through the border is now a "fraction" of what it was last year.

"From the Chinese side, the customs clearing process has trucks spend more time at the border trying to get cleared," he said.

Coal exports have helped shore up Mongolia's struggling economy, but they have been hit by Chinese border restrictions. Trade slowed after customs authorities cracked down on the smuggling of meat and other goods into China last year.

Truck queues stretched as far back as 130 km on one border road for more than a week in October 2017, forcing Mongolia to close it down to clear traffic. Authorities plan to build a new route to cut down on congestion.

China is Mongolia's number-one trade partner, and bought 87 percent of its northern neighbour's total exports in the first two months of the year, including nearly all of its coal. Mongolia's exports to China fell 3.9 percent compared with a year earlier.

Commodities other than coal fared better. Gold trading value more than doubled to $72 million while copper concentrate exports rose 11.5 percent.

Mongolia's trade surplus reached $225.4 million during the first two months, the statistics office said.