A diplomat who sparked a bomb alert on a flight from Washington DC to Denver may have been smoking a cigarette, US security officials have said.

The man was arrested by air marshals on United Airlines flight 663 on Thursday after smoke was smelt while he was in the bathroom.

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

Early reports had said that the man had "tried to set his shoes alight", but NBC News, quoting unnamed security officials, reported that no explosives had been found on the aircraft.

"Two US air marshals aboard asked the man when he emerged what he had been doing and depending on which version you hear, he either said something to the effect that he had been trying to light his shoe, which may have been interpreted literally or it was a sarcastic remark," Al Jazeera's Tom Ackerman, reporting from Washington, said.

"In either case, there was a physical confrontation, according to the FBI."

Diplomat's 'mistake'

US media reports said that the man was Mohammed al-Modadi, the third secretary and vice-consul of the Qatari embassy in Washington.

Ali bin Fahad al-Hajri, Qatar's ambassador to Washington, did not identify the diplomat involved in the incident but denied that he carried out any "threatening activity".

"The facts will reveal that this was a mistake, and we urge all concerned parties to avoid reckless judgments or speculation," a statement said.

"We respect the necessity of special security precautions involving air travel, but this diplomat was traveling to Denver on official embassy business on my instructions, and he was certainly not engaged in any threatening activity."

Authorities said two jet fighters from Buckley Air Force Base were scrambled to accompany the jet for the final 40 miles of its flight to Denver, where it landed safely.

"The flight landed safely 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 travelling public."

The incident came a week after the United States unveiled new security measures subjecting all US-bound plane passengers to screening methods that use real-time intelligence to target potential threats, replacing the mandatory screening of passengers from a blacklist of 14 countries.
  
The measures were announced in the wake of the apparent failed attempt to detonate concealed explosives on a Detroit-bound flight from Amsterdam on Christmas Day.