(Reuters) - FedEx Corp FDX.N, United Parcel Service Inc UPS.N and the U.S. Postal Service said on Friday they are already working to stop traffickers from using their services after U.S. President Donald Trump ordered delivery firms to refuse packages of the powerful painkiller fentanyl from China.

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Trump told those shippers and online retailer Amazon.com Inc AMZN.O to decline deliveries of the synthetic opioid from China, which on Friday announced new retaliatory tariffs on a swath of U.S. 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 at least 3% after the latest salvo in the U.S.-China trade war sent the U.S. stock market lower.

“We work closely with all law enforcement and regulatory authorities to monitor for prohibited substances,” UPS said.

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

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

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

Drug traffickers target U.S. 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.

“The U.S. Postal Service is aggressively working to implement provisions of the STOP Act to keep dangerous drugs from entering the United States from China and other countries,” USPS said in a statement, referring to Synthetics Trafficking and Overdose Prevention Act enacted by U.S. Congress in 2018.

That legislation required the Postal Service to receive advance electronic data (AED) - including the names and addresses of senders and recipients, package contents and other information - on all shipments from China by the end of 2018 and from all countries by the end of 2020.

China’s STOP compliance has lagged and the USPS and U.S. Customs and Border Protection (CBP) have informed China’s postal operator any U.S.-bound shipment without AED may be returned at any time, the postal carrier said.

Meanwhile, USPS and its law enforcement arm, the Postal Inspection Service, continue to work with government and law enforcement agencies to combat the trafficking of illicit drugs like fentanyl, USPS said.

Amazon did not reply to a request for comment.