A passenger managed to slip a gun past a security checkpoint at Atlanta’s Hartsfield-Jackson International Airport earlier this month — but Transportation Security Administration officials insist TSA staff going unpaid during the weeks-long government shutdown were not to blame.

According to the New York Post, the agency confirmed that an unidentified passenger flying out of the world’s busiest airport on January 2 somehow got a firearm past security agents prior to a flight to Tokyo’s Narita International Airport, where he was met by Japanese authorities upon landing.

It was not clear if the gun was loaded.

“TSA has determined standard procedures were not followed and a passenger did in fact pass through a standard screening TSA checkpoint with a firearm at Hartsfield-Jackson Atlanta International Airport on Jan. 2,” the organisation revealed in a statement. “TSA will hold those responsible appropriately accountable.”

But TSA personnel maintained that the security breach was not the result of the government shutdown — which went into its 24th day on Monday, the longest in US history.

“There was not a staffing issue as some are speculating or alluding to in their articles,” the agency’s statement continued.

The Delta Air Lines passenger was cooperative with authorities once he landed in Tokyo, according to the TSA, which referred additional inquiries to airport officials at Narita.

A spokeswoman for Delta Air Lines declined to comment, referring all questions to TSA officials.

The federal agency, according to statistics posted on its website, had a 7.6 per cent unscheduled absence rate on Monday, compared to a 3.2 per cent rate on the same date a year ago.

“Most importantly, security standards remain uncompromised at our nation’s airports,” the agency’s website reads.

“TSA men and women across the country continue to report for work at our nation’s airports to secure aviation. From passengers to airport personnel, and others making their way through terminals across the country, the kind words of support have not gone without notice or positive impact.”

A total of 1.97 million passengers were screened nationwide by TSA officials on Sunday, 99 per cent of whom waited less than 30 minutes.

More than 93 per cent of them had to wait less than 15 minutes to be screened before boarding their planes, according to TSA statistics.

Contingency plans were put into place on Monday at Washington-Dulles International Airport and Hartsfield-Jackson Airport due to workers calling out over inclement weather and an anticipated high volume of traffic, respectively.

Roughly 800,000 federal employees, including TSA security officers, have either been working without pay or have been furloughed since the partial government shutdown began on December 22 after US President Trump and Congress disagreed over $5.7 billion ($A7.9bn) in funding for a wall on the US-Mexico border.

Staffing shortages at two of the country’s major travel hubs — Houston’s George Bush International Airport and Miami International Airport — forced closures of either TSA screening lines or concourses over the weekend, the Washington Post reported.