HONG KONG — China is investigating an American pilot for FedEx on suspicion of smuggling ammunition, in a case that threatens to deepen troubles for a courier already caught in the crossfire of the U.S.-China trade war.

Chinese Foreign Ministry spokesman Geng Shuang told reporters Friday that customs officials in the southern city of Guangzhou detained a U.S. citizen Sept. 12 who was carrying 681 airsoft gun pellets in his luggage and planning to travel to Hong Kong.

The pilot was first identified by the Wall Street Journal this week as Todd A. Hohn, a former U.S. Air Force colonel. Hohn was carrying nonmetallic pellets used in low-power replica air guns in a checked bag and cannot leave China pending the outcome of his investigation, the Journal reported, citing people familiar with the matter.

Hohn is out on bail and under criminal investigation for smuggling ammunition, Geng said Friday.

FedEx confirmed in a statement that a pilot was detained before a commercial flight in Guangzhou. “We are working with the appropriate authorities to gain a better understanding of the facts,” the company said.

China has some of the world’s strictest gun-control regulations, and even many Chinese are not aware of draconian rules governing replica airsoft weapons, which fire pellets that can leave bruises but are seldom deadly. Chinese state media in 2016 covered the case of a man in Fujian province who was bewildered when he was given life imprisonment after ordering 24 replica guns from the Internet.

Hohn’s case adds another pressure point for FedEx in China, where the company faces serious political risks. Chinese officials have opened investigations into the express service since June, and state media have suggested that the company could be placed on a blacklist of “unreliable” foreign firms after Huawei, the Chinese technology giant, accused FedEx of misrouting parcels and potential foul play.

China’s proposed unreliable-entities list appeared to be a response to Washington’s decision to ban U.S. companies from doing business with Huawei after the U.S. government added the Chinese technology firm to a Commerce Department blacklist.

FedEx said this week that its business outlook for the year has darkened dramatically amid the U.S.-China trade dispute.

Chinese authorities have been investigating FedEx in recent weeks for illegally shipping weapons, including knives, to Hong Kong, according to the state-run Xinhua News Agency. FedEx has said the package containing knives never left its origin city — coincidentally, Guangzhou, where Hohn was detained.

All summer, China has accused the United States of covertly fomenting protests and violence in Hong Kong, but has not provided substantial evidence.

Hohn, who left the Air Force in 2017, had passed through security and was waiting in an executive lounge for a Cathay Dragon flight home to Hong Kong when he was detained, the Journal reported.

A protracted investigation into Hohn could add strain to bilateral relations at a time when Western countries, including Canada, Australia and the United States, have accused China of seizing foreign nationals on murky or trumped-up charges to apply political pressure to their governments.