US rapper Juice WRLD was given anti-overdose medication by a US federal agent just before his death in Chicago, authorities have confirmed.

Key points: Chicago police responded to a tip-off about guns and drugs on the plane

Chicago police responded to a tip-off about guns and drugs on the plane Suspected illegal drugs, prescription drugs and three guns were found

Suspected illegal drugs, prescription drugs and three guns were found Juice WRLD had reportedly taken an opioid painkiller and went into convulsions

The agent, who had been searching the rapper's private plane for drugs and firearms, administered the opioid antidote Narcan to him after he went into convulsions.

The plane carrying the 21-year-old, who was named top new artist at the 2019 Billboard Music Awards in May, and about a dozen other people had landed at Chicago's Midway International Airport at around 2:00am on Sunday (local time).

Federal agents and city police officers, acting on a tip that a private plane coming from Los Angeles was carrying guns and drugs, were waiting at the airport, Chicago police spokesman Anthony Guglielmi said.

During a search in a private hangar, a drug-sniffing dog made a "positive alert" on luggage carts that were loaded with bags from the plane, Mr Guglielmi said.

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Inside, they found multiple bags of suspected marijuana, several bottles of prescription cough syrup, three guns, armour-piercing bullets and a high-capacity ammunition magazine, he added.

The rapper, whose real name was Jarad Higgins, went into convulsions during the search.

One of the agents administered Narcan after the rapper's girlfriend said he had been taking Percocet, an opioid painkiller, Mr Guglielmi said.

Juice WRLD woke up but remained incoherent, the Chicago Tribune reported.

He was rushed to a local hospital where he was pronounced dead at about 3:15am.

Juice WRLD had celebrated his birthday on a luxury boat on Sydney Harbour only days before.

The rapper went into convulsions when police searched his private plane. ( Invision/AP, File: Owen Sweeney )

An autopsy was completed on Monday but the toxicology test results were still pending, the medical examiner's office said.

Foul play is not suspected in his death.

AP/ABC