According to reporting from Reuters, officials in at least eight states have also told residents to not put the seeds in the ground. The US government was informed that in recent days, seeds apparently from China arrived in mailboxes of people who did not order them. Officials fear that the seeds could be invasive species that could threaten crops or livestock.

"At this point in time we don't have enough information to know if this is a hoax, a prank, an internet scam or an act of agricultural bioterrorism," Ryan Quarles, Kentucky's agriculture commissioner, said in a video posted on the department's website.

Photos of packages that state agriculture departments posted on social media show seeds of different sizes, shapes and colours that arrived in white or yellow envelopes. State officials said some packages were labelled as jewellery and may have contained Chinese writing.

The Chinese Embassy in Washington, DC, did not immediately respond to a request for comment.

The USDA said it is "aware that people across the country have received unsolicited packages of seed from China in recent days." The agency is working with the Department of Homeland Security and states to protect US agriculture and prevent the unlawful entry of prohibited seeds, according to a statement.

State officials have asked recipients to secure the seeds in sealed plastic bags until they are picked up by authorities.

Police in Whitehouse, Ohio, said it appears the seeds are tied with a scam in which vendors ship inexpensive products to unwitting receivers and then submit positive reviews on e-commerce websites on the receiver's behalf.

"Although not directly dangerous, we would still prefer that people contact us to properly dispose of the seeds," the police said on Facebook.

The Washington State Department of Agriculture said on Facebook that the shipments are "agricultural smuggling." It asked recipients to save them for the USDA because "they may be needed as evidence."

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