At least four people have been killed and 18 injured in a mass shooting in Birmingham, Alabama, police say.

“Multiple shooters fired multiple shots on a group of people” late on Saturday in the Five Points South area of the city, Birmingham police officer Truman Fitzgerald said.

Officers found the bodies of two men and one woman at the scene, while a third man later died of bullet wounds in hospital, Birmingham Police said.

Detectives are investigating whether the gunmen walked up to the victims or drove by, Mr Fitzgerald said. No suspects have been arrested.

He added that they believed the shooting was "not random and stemmed from an isolated incident where multiple victims were caught in the crossfire".

Detectives are working to identify who was the intended target or targets of the attack, Mr Fitzgerald also said.

They are also pressing to find the shooters. The police said in a statement that they are working with the FBI and other federal agencies, are offering a $5,000 reward for information and have opened a web portal for submitting photos and videos of the incident.

The Five Points South district is known for its nightlife. The shooting occurred on Magnolia Avenue, Mr Fitzgerald said.

Witnesses who were queuing outside a hookah and cigar lounge on Magnolia Avenue at the time told local news site Al.com that some of the gunfire sounded as though it came from a gun converted to be fully automatic.

Earlier on Sunday, Mr Fitzgerald had told reporters there were "dozens of gunshot victims" after the incident.

Birmingham Mayor Randall Woodfin blamed "Glock switches" - devices that can be attached to handguns to make them fire automatically - for the violence, posting on social media on Sunday that they "are the number one public safety issue in our city and state".

"Converting a semi-automatic weapon into a fully automatic weapon that discharges all bullets within seconds doesn’t belong on our domestic streets," he wrote, adding that the city does not have the power to outlaw Glock switches, only the state.

There have been more than 400 mass shootings across the US so far this year, according to the Gun Violence Archive, which defines a mass shooting as an incident in which four or more people are injured or killed.