The USDA says the deliveries are likely part of a “brushing scam.”

Officials in at least 15 states across the country have issued warnings for residents not to plant unsolicited packages of seeds delivered through the mail and labeled as coming from China.

The United States Department of Agriculture, the Department of Homeland Security, and other state and federal agencies are investigating the packages that have shown up in mailboxes across the country.

The thin light gray packaging has a shipping label that appears to come from Suzhou, a city west of Shanghai, China, and identifies the contents as a piece of jewelry like a ring or earrings. Inside, however, is a clear plastic bag of unidentified seeds.

“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,” USDA officials said.

“Do not plant seeds from unknown origins,” they added.

Officials are collecting the seed packages from recipients and will test their contents to determine if they contain anything that could be of concern to the country’s agriculture or the environment, officials said.

Wang Wenbin, spokesperson for China’s Ministry of Foreign Affairs, confirmed on Tuesday that the labels are forged and says the Universal Postal Union “strictly” prohibits sending and receiving seeds through the mail.

“After confirming with China Post, the China Post face-to-face slips on this batch of mails are forged, and there are many errors in the layout of the face-to-face slips and information items,” Wenbin said during a press conference. “China Post has negotiated with U.S. Post to return these fake mails to China for investigation.”

Since Sunday, residents in cities including Nashville, Philadelphia, Cincinnati, Tampa and Minneapolis have reported receiving the mysterious packages.

The USDA urges anyone who receives an unsolicited package of seeds to immediately contact their state’s plant regulatory official or the plant health director of their state’s Animal and Plant Health Inspection Service.

“Please hold onto the seeds and packaging, including the mailing label, until someone from your state department of agriculture or APHIS contacts you with further instructions,” officials said.

ABC News’ Kirit Radia contributed to this report.