WASHINGTON (REUTERS) - FedEx and United Parcel Service said on Friday (Aug 23) they already work closely with law enforcement to stop illegal shipments from entering the country after US President Donald Trump ordered delivery firms to refuse packages of the powerful painkiller fentanyl from China.

Trump told FedEx, UPS, online retailer Amazon.com and the US Postal Service (USPS) to decline deliveries of the synthetic opioid from China, which on Friday announced new retaliatory tariffs on a swath of US goods.

"I am ordering all carriers, including Fed Ex, Amazon, UPS and the Post Office, to SEARCH FOR & REFUSE all deliveries of Fentanyl from China (or anywhere else!)," Trump said on Twitter.

"President Xi said this would stop - it didn't," he tweeted, referring to China's President Xi Jinping.

Fentanyl is 50 times more powerful than heroin and is contributing to the nation's deadliest drug crisis in history.

Shares of FedEx, UPS and Amazon were each down more than 3 per cent after the latest salvo in the US-China trade war that sent the whole US stock market lower.

"UPS takes a multi-layered approach to security and compliance to identify and prevent delivery of illegal fentanyl and other illicit substances," the company said.

Rival FedEx in a statement said it "already has extensive security measures in place to prevent the use of our networks for illegal purposes."

Amazon and USPS did not reply to a request for comment.

Private delivery companies like UPS and FedEx electronically track packages, which has deterred some drug dealers, who instead have exploited the less sophisticated USPS tracking system.

"The most common distribution medium is via the US Postal Service," the US Treasury said in a statement on Wednesday, when it announced sanctions against a trio of Chinese nationals accused of trafficking fentanyl.

US Customs and Border Protection (CBP) port officers are responsible for the inspection and interdiction of packages and cargo at all international mail inspection at USPS and private shipping company facilities, according to a May 2018 report released by then-US Senator Claire McCaskill.

Drug traffickers target US ports of entry and international mail centers, where parcel and vehicle inspections are limited due to staffing shortages and other constraints, experts and officials said.

CBP representatives did not reply to requests for comment on Friday.