Farmers in the United States have been warned not to plant any seeds that turn up in their mailbox out of the blue.
"USDA is aware that people across the country have received suspicious, unsolicited packages of seed that appear to be coming from China," the department said in a statement on Tuesday (local time).
The department's Animal and Plant Health Inspection Service (APHIS) said it was working with the Department of Homeland Security and other federal agencies to investigate the packages.
"Pls don't plant seeds from unknown origins!" the USDA added in a tweet.
Anyone receiving an unsolicited package was urged to hold on to the seeds and packaging, including the mailing label, and contact APHIS.
"At this time, we don’t have any evidence indicating this is something other than a 'brushing scam' where people receive unsolicited items from a seller who then posts false customer reviews to boost sales," the department said.
"USDA is currently collecting seed packages from recipients and will test their contents and determine if they contain anything that could be of concern to US agriculture or the environment."
New York Commissioner of Agriculture Richard Ball said the packages containing seeds were falsely marked as containing jewellery, CBS News reported.
Residents of at least eight states have so far reported receiving the suspicious packages.
A spokesperson for the Ministry for Primary Industries (MPI) told Newshub unsolicited seeds had been intercepted at the border here by Biosecurity New Zealand for "a number of years".
MPI was working with e-commerce platforms to stop the practice, the spokesperson said.
"The unsolicited seeds are often incorrectly declared or do not meet other biosecurity requirements. In such cases, our officers seize and destroy the seeds.
"We ask anyone who receives an unsolicited seed packet in the post to destroy it or contact our pests-and-diseases hotline on 0800 80 99 66."