Amazon has barred foreign sales of seeds in the United States, following unsolicited mystery packets being shipped to Americans, mostly postmarked from China.

The retail giant's policy on plant and seed products, which was updated on September 2, prohibits the sale of seeds imported from outside the United States or sold by non-US residents. The new policy extends to both plants and plant products.

"Moving forward, we are only permitting the sale of seeds by sellers who are based in the US," Amazon said in a statement.

Sellers who don't comply with Amazon's policies could have actions taken against their account, which include listing cancellations, suspended or forfeited payments or being banned from selling on the site. It's unclear if the seed ban will extend to other countries.

Amazon's new rule comes after all 50 states were issued warnings by the US Department of Agriculture in July, after mystery packages of seeds had been sent to households in the US with postmarks from China. Some packages were mislabeled, specifying they included jewelry or beads, despite only containing a clear, plastic package of seeds.