The landlocked northeast Asian country has become one of China's biggest coal suppliers this year following a ban on imports from North Korea and restrictions on deliveries to some smaller Chinese ports.

But a backlog led to the queue of trucks on the main road to China, with customs clearance at the Gants Mod border crossing said to be taking as long as a month, up from the normal period of two weeks, according to Mongolia's official Montsame news agency.

"There was a problem at the border, so at the end of July and beginning in August, there were 400 to 500 trucks passing through the border gate," said D. Davaasuren, state secretary for the Ministry of Foreign Affairs, at the briefing, without giving more detail.

Davaasuren said Mongolia had discussed the matter with Chinese officials and around 990 trucks entered China from Mongolia on Tuesday, closer to normal levels.

Last week, the country mobilized to improve carrying capacity at border checkpoints and also promised to negotiate with Chinese officials to ease the bottleneck, Montsame reported.

Coal trucks were forced to wait two or three days before they were even permitted to unload for inspections at the border, according to the website of Chinese industry consultants sxcoal.

Though Mongolia is currently upgrading its detection equipment at border crossings, an official familiar with the situation rejected claims that the delays were caused by technical problems. The official said operations at the border might have been disrupted as a result of lower staffing levels during Mongolia's traditional summer Naadam festival.

China's General Administration of Customs said on Wednesday that 2.26 million tonnes of non-lignite Mongolian coal were imported in July, up 32.4 percent from a year ago.

Mongolia has been China's third-biggest source of coal imports over the first seven months of this year, with total shipments reaching 20.85 million tonnes, up 73 percent compared to last year.

With a long-awaited railway connection still unfinished after investment dried up, all coal from mines in Mongolia's Umnugovi province abutting China are delivered to the border via one single highway, making it vulnerable to disruptions.

Exporters of other commodities have not reported any issues at the border. Copper shipments from the Oyu Tolgoi mine run by Anglo-Australian miner Rio Tinto, which uses the same road and border crossings for exports, have not experienced any new delays, an official at the mine told Reuters.