Washington DC - The US Congress on Thursday passed a bill imposing harsher penalties on fentanyl traffickers, with lawmakers from both parties anxious to crack down on an opioid that has driven an epidemic of deadly overdoses.

The Senate-passed bill – which delivers on a key election pledge of President Donald Trump to get tough on drug smuggling – was rubber-stamped by the House on a 321-104 vote, with Democrats providing all but one of the no votes.

The Justice Department says 75,000 Americans die each year because of fentanyl, making it the number one cause of death for people between the ages of 18 and 34 in the US.

The synthetic opioid is more potent than heroin and much cheaper to produce.

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"More Americans die of drug overdoses each year than the number of Americans who died in the entirety of the Vietnam War," Republican Senate Majority Leader John Thune said ahead of the vote.

The HALT Fentanyl Act places copycat variations of fentanyl – often sold by traffickers – on the government's list of most dangerous "Schedule 1" drugs alongside the original substance.

Lab-created fentanyl alternatives were reclassified to "Schedule 1" on a temporary basis seven years ago, but the vote makes the change permanent.

Trump has made halting the flow of fentanyl one of his priorities, even announcing it as a justification for import tariffs on Mexico and Canada.

But opponents said the new law – rather than tackling overdoses – would simply repeat the mistakes of the so-called "War on Drugs."

The Leadership Conference on Civil and Human Rights, a coalition of more than 240 US rights organizations, said in a statement that the measure would "exacerbate pretrial detention, mass incarceration, and racial disparities in the prison system."

"Beginning in the 1980s, draconian drug laws with harsh mandatory minimums and their resulting enforcement under the banner of the 'war on drugs' fueled skyrocketing prison populations," it said.