Packages mailed from China and Hong Kong will now be subject to formal customs entry, and will no longer be eligible for administrative exemption from duties, according to a notice by the US Customs and Border Protection released Monday night in the US.

The rules come into effect after US tariffs against Chinese goods came into effect midnight of Tuesday to "address the synthetic opioid supply chain in the People's Republic of China," CBP said.

Among the exemptions that will be removed are goods eligible for de minimis clearance, CBP said.

Customs and Border Protection also informed shippers on how to handle such shipments in accordance with tariff guidelines on Chinese goods.

"As we transition to execution of the Executive Order, it is extremely important that the trade maintain awareness of the shipments they are responsible for and are aware of the messaging that they are receiving from CBP," according to the notice.