US federal agencies are investigating suspicious and unsolicited packages of seeds that have been arriving at addresses across America and appear to come from China.

Anyone who receives the packages should hold onto the seeds and packaging and immediately contact officials, the US Department of Agriculture said in a statement.

The USDA doesn’t have any evidence to indicate this is something other than a brushing scam, where a seller sends unsolicited packages so they can post false customer reviews online and boost sales, it said.

The department is collecting the seed packages and will test their contents for anything that could be of concern to US agriculture or the environment, it said.

Chinas foreign ministry on Tuesday said that China Post has strictly followed regulations that ban the sending and receiving of seeds. China Post has talked to its US counterparts about returning parcels to the Asian nation so it can investigate further, Foreign Ministry spokesman Wang Wenbin said.