American Airlines removed a passenger who boarded a flight in Dallas on Thursday while wearing a gas mask - causing panic among the other fliers.

The incident took place at Dallas Fort Worth International Airport, where American Flight 2212 was preparing to depart to Houston in the evening.

But passengers grew alarmed at the unusual sight - a man boarded wearing a gas mask over his face and a black beanie covering his head.

'Just FYI flight 2212 to Houston was delayed an hour because you let this guy on the plane wearing a gas mask,' Joseph Say wrote on Twitter in a tweet directed at the airline as well as news media outlets.

Passengers aboard an American Airlines flight waiting to take off from Dallas Fort Worth International Airport on Thursday were alarmed when they saw a man board wearing a gas mask and a black beanie

'Just FYI flight 2212 to Houston was delayed an hour because you let this guy on the plane wearing a gas mask,' Joseph Say wrote on Twitter

'This then panicked people on the plane and we had to wait for him to be escorted off.'

Say told ABC 13 TV: 'I had a seat in the back. I was sitting there. I was talking to the lady next to me.

'We heard a little bit of commotion. I looked up and saw a guy coming onto the plane wearing a full gas mask, which was kind of odd.

'He didn't have a filter, which I thought was more strange. Immediately, people start talking in the back of the plane. They were worried.

'Most people wanted him off the plane.'

Say said passengers wanted to know why the unidentified person was wearing a gas mask.

A flight attendant then came over to the man and asked him to remove the gas mask, but he refused, according to Say.

After the flight crew called security, the man left the aircraft.

'What we heard from the lady sitting next to him was he said he wanted to make a statement,' Say said.

'I don't know what the statement was. I'm not sure what his goals were. To me, it seemed inconsistent.'

American Airlines said that the man was removed from the plane and was rebooked on the next flight to Houston. The image above shows an American Airlines Boeing 737 passenger jet taxiing at Dallas Fort Worth International Airport in September 2017

Say said: 'You couldn't see his face. You couldn't identify any features on him.

'People were worried he had sneaked something on-board and that he had the mask for his own safety.'

After a 50-minute delay, the plane departed Dallas Fort Worth and landed at Bush Intercontinental Airport at 9:07pm local time.

It is not known if the man wearing the gas mask was charged with a crime.

An American Airlines spokesperson told ABC 13 TV: 'The passenger was rebooked on the next flight to Houston, and flew without the mask on yesterday evening.'

The airline said that the masked man had flown into Dallas Fort Worth from Los Angeles, though it is not known if he wore the mask during that flight.