A passenger on a United Airlines flight from Washington to Denver sparked a security alert on Wednesday after he was apparently caught smoking in the toilet and made a remark that was perceived as a threat.

Officials said a Qatari man was in custody but the incident did not appear to be serious.

United Airlines Flight 663, which originated from Washington's Reagan National Airport, landed safely in Denver about 9 pm EDT (0100 GMT on Thursday) after two F-16 fighter jets intercepted the Boeing 757 aircraft, according to the North American Aerospace Defense Command responsible for safeguarding US airspace.

"It looks like the individual in question was perhaps smoking in the lavatory and might have made an unfortunate remark" when confronted by airline personnel, a US official said.

US officials have been on heightened alert and have ramped up security substantially since Christmas Day last year after a Nigerian man tried but failed to blow up a flight from Amsterdam to Detroit with a bomb hidden in his underwear.

ABC News, citing federal law enforcement officials, had reported earlier that US air marshals subdued a Qatari man who authorities say tried to "light his shoes on fire" on the flight, sparking fears of another attack.   

CNN said no explosives were found on board and identified the man as a mid-level Qatari diplomat. When asked why he was in the toilet so long, he "said something about lighting his shoe on fire," the network reported.   

US officials could not confirm that report. The plane was surrounded by security vehicles when it arrived at Denver International Airport.   

"Law enforcement and TSA have responded to the scene and the passenger is currently in custody," the Transportation Security Administration said in a statement. "All steps are being taken to ensure the safety of the traveling public."    

There were 157 passengers aboard the plane and six crew members, according to United Air. Attempts to reach the Qatari Embassy in Washington were not immediately successful.   

US officials have tracked the December plane incident back to al Qaeda in the Arabian Peninsula. This has led them to step up efforts to combat the militant group in Yemen where the Nigerian man told investigators he trained and obtained the bomb. -Reuters