Donald Trump has congratulated U.S. Customs and Border Protection officials after they made their biggest ever fentanyl bust, seizing a quantity large enough to kill 100million people.

CBP officials said they captured nearly 254 pounds (114kg) of the deadly synthetic opioid from a secret compartment inside a truck carrying a load of Mexican produce heading into US at the Nogales border crossing in Arizona.

The drugs, with a street value of about $3.5million, were found on Saturday morning, hidden in a compartment under the rear floor of a tractor-trailer.

The president tweeted his praise for the officials on Thursday, writing; 'Our great US Border Patrol Agents made the biggest Fentanyl bust in our Country's history. thanks, as always, for a job well done!'

Agents also seized nearly 395 pounds (179kg) of methamphetamine with a street value of $1.18million, Nogales CBP Port Director Michael Humphries said.

After a scan during secondary inspection indicated 'some anomalies' in the load, the agency's police dog team alerted officers to the presence of drugs, Humphries said.