China as placed the United States on a list of countries which must take precautions that its shipments are mosquito-free to prevent the spread of the Zika virus.

China first promulgated import rules on Zika on March 2, but only added the U.S. to the list last week.

The move is expected to delay exports. U.S. exporters and government officials are unclear how to comply with the new regulations.

The Chinese General Administration of Quality Supervision, Inspection and Quarantine, released by the Mediterranean Shipping Company (MSC), stated that “with immediate effect, it means that there is a need to provide a certificate of extermination of mosquito. If no certificate is provided, the buyer must fumigate the cargo at arrival at port of destination.”

The Agriculture Transportation Coalition, a U.S. trade group, said in a statement that it expects that the new rules will prevent U.S. exporters from delivering goods to China in an affordable and timely manner.

The new rules also do not mention which modes of transportation the new rules are applicable to, raising questions on how air cargo carriers will comply. The announcement states that “vehicles and containers from the countries and regions shall be subject to effective anti-mosquito treatment…”

In Chinese agency stated that “port authorities shall adopt effective anti-mosquito measures to eliminate mosquito breeding sites, reducing the mosquito density of the port. Inspection and quarantine authorities shall strengthen port health supervision, to prevent the spread of mosquitoes in the port Zika virus and other infectious diseases.”

Zika is most commonly transmitted through the Aedes aegypti and albopictus mosquitoes. The Centers for Disease Control and Prevention reported that over 416,000 cases of Zika virus have been reported in 38 countries in the Western Hemisphere. In the United States, most of the 6,500 reported cases have come from U.S. territories, especially Puerto Rico.