A US airliner made an emergency landing in Texas on Tuesday after the pilot, shouting about Al-Qaeda and bomb threats, had to be restrained by passengers, officials and reports said.

The JetBlue flight from New York to Las Vegas, Nevada was diverted to Amarillo, Texas after the incident, which the airline said was due to a medical issue involving the captain.

The captain went to a restroom just outside the cockpit, and when he emerged began shouting ‘Iraq, Al-Qaeda, terrorism, we're all going down!’ according to the Amarillo Globe-News newspaper.

"It was a little scary. He was pretty freaked out," passenger Heidi Karg said. "When they were trying to calm him down... they had to restrain him and a bunch of male passengers ran to the front of the plane to subdue him."

Gabriel Schonzeit, who was seated in the third row, said: "He started screaming about Al-Qaeda and possibly a bomb on the plane and Iraq and Iran and about how we were all going down."

Amateur video showed chaos at the front of the plane as male passengers scrambled to help restrain the pilot, who could be heard shouting hysterically after being pulled away from trying to get back into the cockpit.

The captain was described as a "very large man," but passengers on the flight - many of whom happened to be security professionals heading to a conference in Las Vegas - managed to pin him down.

"The pilot was a very large man, bigger than me," said Schonzeit, who is 6 feet, 3 inches tall and weighs 250 pounds.

"Two flight attendants tried to subdue him, and then seemingly out of nowhere about six or seven large guys stormed to the front of the plane and wrestled (him) to the ground," passenger Laurie Dhue said.

They "had him subdued in a matter of moments," she added. "It was really like something out of a movie."

The Federal Aviation Administration (FAA) said the plane's co-pilot worried that the captain was behaving erratically.

"The co-pilot became concerned that the captain exhibited erratic behavior during the flight. The captain had exited the cockpit during the flight, after which the co-pilot locked the door," said an FAA statement.

"When the captain attempted to enter the locked cockpit, he was subdued by passengers. After the flight landed safely, local law enforcement secured the pilot .. and he was transported by ambulance for medical evaluation."

A JetBlue statement said that roughly three and a half hours into the flight, "the pilot in command elected to divert to Amarillo, Texas for a medical situation involving the captain.

"Another captain, traveling off duty, entered the flight deck prior to landing at Amarillo, and took over the duties of the ill crew member, who was taken to a local medical facility in Amarillo.

TV pictures showed the pilot, strapped to a stretcher, being wheeled off the aircraft.

"It seemed like he went crazy," passenger Schonzeit told the Globe-News.

"We all knew in the front rows if he got back in the cockpit, we were going down."

Several hours later passengers boarded another plane to complete their journey to Las Vegas.

Another passenger, Tony Antolino, hailed the co-pilot as a ‘hero.’

"He had the sense to recognize that something was going horribly wrong and he was able to persuade the pilot out of the cockpit," he told CNN.