The lightweight championship clash will top a bill of six fights set to take place on the US Flag Day holiday and the 80th birthday of President Donald Trump, who is an avowed UFC fan.

The June 14 event has been dubbed "UFC Freedom 250," in reference to the United States' 250th anniversary celebrations this summer.

Topuria is one of the UFC's top rated fighters and a double-weight champion who has won all 17 of his fights.

A former featherweight champion, he earned the lightweight belt last year by knocking out Brazil's Charles Oliveira before temporarily stepping away from the sport for personal reasons.

Born in Germany to Georgian refugee parents, Topuria moved to Spain as a teenager, and has both Georgian and Spanish citizenship.

In Topuria's UFC absence, Arizona native Gaethje won an interim lightweight title, but will now have a chance to capture the undisputed championship.

The 37-year-old American has a 27-5 record in UFC.

Trump has said a stadium for more than 100,000 people would be constructed in front of the White House for the fight, although organizers have since said the event is expected to host some 3,000 to 4,000 people at the heart of Washington DC.

Large screens to view the event are expected to be raised in other places near the White House for larger crowds to watch the show.

The event was originally announced for July 4, which is Independence Day in the US, before being moved to Trump's birthday.

UFC is the largest and most successful organization in the burgeoning world of MMA, a blend of martial arts disciplines like jiu-jitsu, kickboxing, boxing and wrestling.

Bouts take place in an eight-sided ring -- dubbed "The Octagon" -- bounded by a chainlink fence.

With few exceptions -- such as eye-gouging -- male and female fighters are allowed to employ almost any technique to attack their opponent.

The sport's popularity with young men -- a key demographic in the 2024 US election -- and Trump's long association with the UFC have made the president a regular fixture at some of its more high-profile events, where he is greeted like a rock star.

Its brutal nature and high injury rate mean the sport is controversial, with doctors decrying the potential for brain damage amongst fighters who are repeatedly hit in the head, though it has gained increasing mainstream acceptance in recent years.