image copyrightReutersimage captionInvestigations into possible breaches of the law on flights have reached record levels in the US
Airlines have reported about 3,000 cases of unruly passengers to US aviation authorities this year as bad behaviour on flights takes off during the coronavirus pandemic.
The Federal Aviation Administration (FAA) said 2,300 of those reports were about people who refused to wear masks.
Videos of rows over mask requirements on flights have been going viral.
This year has also seen a record number of possible violations of the law on flights.
Passengers found guilty of unruly or dangerous behaviour can face fines or possible jail time.
The FAA said it had investigated the highest number of potential breaches of the law since records began in 1995.
The agency said some 394 cases of passengers allegedly "interfering with the duties of a crew member" had been reported as of 25 May.
This is twice as many as the whole of last year, when 183 cases were investigated.
"In a typical year the agency will end up taking this type of enforcement action in about 100 to 160 enforcement cases so it's nothing new," said Steve Dickson, the chief of the FAA, in an interview with ABC News last month. "What really is new is the volume that we're seeing right now."
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In January, the administration of US President Joe Biden issued an order that made it national law to wear a mask on commercial airlines and other modes of transport.
The order was issued to curb the spread of Covid-19 in the US, which has recorded the most infections in the world.
Following this, the US Centers for Disease Control and Prevention (CDC) ruled that air passengers must wear face masks, unless they have an exemption.
image copyrightEPAimage captionAir passengers are required to wear masks by law in the US, unless they have an exemption
The FAA says it is enforcing a zero-tolerance policy towards passengers who cause disturbances or fail to obey the mask requirement on flights.
As part of this policy, the agency unveiled plans to impose stricter penalties against unruly airline passengers in January this year. The agency said passengers could face fines of up to $35,000 (£24,700) and imprisonment.
"The FAA has seen a disturbing increase in incidents where airline passengers have disrupted flights with threatening or violent behaviour," the agency said.
On Monday, the FAA announced actions against four passengers under these tougher rules, including one woman on a JetBlue flight accused of putting her finger in her nose and swearing at a flight attendant who asked her to put on her mask, forcing the pilot to divert.
Other in-flight incidents have made headlines in recent months.
One saw a passenger fined $52,500 for trying to open the cockpit door and hit a flight attendant in the face twice on a Delta Air Lines flight in December.
Last month, a Southwest Airlines passenger was caught on camera punching a flight attendant after an apparent dispute over mask policy turned violent.
media captionFlight attendant attacked by passenger in the US