A second American Airlines flight has been forced to make an emergency landing after passengers and crew fell ill on board.

Miami-bound flight 904, which left Rio de Janeiro on Friday evening (NZT) carrying 203 passengers and 14 crew members, was diverted to Brasilia after four people became ill, the Daily Mail Online reported. Three of those were crew.

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"The aircraft landed safely at Brasilia at 12.37am and was met by paramedics who evaluated the passenger and crew members. None requested further medical attention," an American Airlines spokesman told the Daily Mail Online.

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"Our maintenance team is currently inspecting the aircraft and performing a thorough maintenance check."

The incident occurred just a day after another American Airlines flight heading to Los Angeles returned to London's Heathrow Airport after 15 people on board fell ill.

Airline officials would not say whether there was a link between the two incidents.

One passenger on board fight 904 told the Daily Mail Online that she saw people "pass out" from the mystery illness before the plane landed.

"We are currently still in Brasilia, where we made an emergency landing because crew members and passengers passed out and became unwell," the woman wrote in an email.

"It sounds exactly like what happened on the other flight, with us not being allowed off the plane for two hours.

"As you can imagine, frustrating chaos followed and most people are stuck here for 48 hours. One of the crew said something about oven cleaner reacting badly."