Five people died and eight are in hospital after a gunman opened fire inside Fort Lauderdale-Hollywood International Airport in Florida.

The gunman was taken into police custody shortly after the shooting, which occurred at 12.55pm local time (4.55am AEST).

The shooting took place in the baggage claim area in Terminal 2, authorities said, sending people scrambling through the terminals and across the airfield, shutting down all flights as police and paramedics flooded the scene.

Broward County Commissioner Chip LaMarca said the shooter arrived on a flight with a checked gun in his baggage.

"He claimed his bag and took the gun from baggage and went into the bathroom to load it. Came out shooting people in baggage claim," La Marca wrote on Facebook.

The suspect in custody has been named by Senator Bill Nelson as Esteban Santiago, a 26-year-old US citizen born in New Jersey but living in Alaska. Senator Nelson said he was carrying military identification.

Esteban reportedly travelled to Fort Lauderdale from Anchorage, Alaska, stopping over in Minnesota. 

He was being questioned by a team of FBI agents and sheriff's deputies, according to Sheriff Scott Israel. He said it was too early to determine a motive.

"He [the suspect] is unharmed. No law enforcement fired any shots," he said.

Witnesses told MSNBC that the shooter was wearing a Star Wars T-shirt and was shot by police as he attempted to reload.

Eyewitness John Schlicher described the shooter as a "slender man" who was "directly firing at us" while passengers waited for their bags to come off the carousel.

"I put my head down and prayed," he told MSNBC, adding that his wife gave first aid to someone who had been shot in the head.

His mother-in-law used her sweater to tend to another victim but it turned out that victim was already dead, Schlicher said.

The shooter reloaded once for a second burst of shooting, he said, but he could not say how many bullets were fired.

The airport has been evacuated and all flights to and from the airport have been cancelled.

Less than three hours after the gunman was taken into custody, panic gripped passengers again following unconfirmed reports that additional shots had been heard at a second terminal and police were searching for a possible second active shooter.

The Broward Sheriff's office confirmed that an "active search" was underway, while the Transportation Security Administration ordered people to "shelter in place", tweeting "Active shooter at #FLL".

In another Facebook post from the airport, La Marca said that police were searching the parking garage for "a possible second active shooter". 

Sheriff Israel told reporters at a press conference at the airport that the suspect did not appear to have acted with anyone else.

"At this point, it looks like he acted alone," Israel said.

However, he added that "this scene is considered fluid and active" as police search the rest of the airport.

Mark Lea, another eyewitness, told MSNBC "there was no rhyme or reason to it."

"He didn't say anything, he was quiet the whole time, he didn't yell anything," Lea said.

The 53-year-old financial adviser from Minneapolis told another news outlet he was in the baggage claim when a man came in and started shooting. 

"I was dodging bullets and trying to help people get out of the way," Lea told NBC News.

"At first we thought it was firecrackers," he said. "Everyone started screaming and running. The shooter made his way down through baggage claim. He had what looked like a 9mm and emptied his entire clip. People were trying to run."

Lea told ABC News that he was "about 20 feet away" when the gunman was arrested.

"Once he was out of ammunition, he threw the gun on the ground and he lay on the ground spread eagle. He was basically waiting for the officers to come get him," Lea said.

Florida Governor Rick Scott described the shooting as "a senseless act of evil".

"It's horrible what happened here," he told a press conference at the airport. "This is a senseless act of evil and we are going to hold whoever did this accountable."

He said he had "reached out" to president-elect Donald Trump and vice president-elect Mike Pence but it was not "the time to be political".

"The biggest thing to do now is to mourn for everyone who lost their lives and pray for those who are still fighting for their lives," he said.

In the US, travellers are allowed to bring firearms on flights, as long as they are unloaded, locked in hard-sided containers and are in checked baggage, according to the TSA. Ammunition can also be placed in checked baggage, but are prohibited from being taken in carry-on, along with magazines, bolts and other gun parts. 

A spokesperson for the Department of Foreign Affairs and Trade said the Australian Embassy in Washington was making enquiries to determine whether any Australians have been affected by the shooting.

It advised that people concerned about Australians who may be in the region should attempt to contact them directly.

If unsuccessful, the Department of Foreign Affairs and Trade's Consular Emergency Centre can be contacted on 1300 555 135 in Australia or +61 2 6261 3305 from overseas.