People dressed in costumes chant as they approach a gate to Area 51 on Friday. (Reuters)

The US military has been forced to apologise for tweeting that it would use stealth-bombers on “millennials” who try to storm Area 51.

More than 2 million people signed up to a Facebook event recently that encouraged attendees to visit the top secret base in Nevada.

But only a few thousand UFO enthusiasts turned up on Friday to the facility, which is rumoured to contain secrets about aliens.

As hordes of enthusiasts turned up, the PR arm of the US military tweeted: “The last thing #Millennials will see if they attempt the #area51raid today” with a picture of military officers in front of a stealth bomber.

View photos Thousands of tourists marched to the gates of the famous facility this week. (Reuters) More

Shortly afterwards the tweet was deleted and the unit, called the Defence Visual Information Distribution Service (DVIDS), apologised, saying it “in no way” reflects their stance.

“Last night a DVIDSHUB employee posted a tweet that in NO WAY supports the stance of the Department of Defense,” the unit later said. “It was inappropriate and we apologise for this mistake.”

Around 1,000 people visited the facility’s gates on Friday and at least six were arrested by police.

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The Storm Area 51 invitation spawned festivals in the tiny nearby towns of Rachel and Hiko, more than two hours’ drive from Las Vegas.

Lincoln County Sheriff Kerry Lee estimated late on Thursday that about 1,500 people had gathered at the festival sites, and more than 150 travelled several additional miles more on bone-rattling dirt roads to get within selfie distance of the gates.

Millions of people responded to the joke Facebook event, which called for people to run into the remote US Air Force test site. Area 51 has long been the focus of UFO conspiracy theories.

“They can’t stop all of us,” the post said. “Lets see them aliens.”

The military responded with stern warnings that lethal force could be used if people entered the Nevada Test and Training Range, and local and state officials said arrests would be made if people tried.

“It’s public land,” the sheriff said. “They’re allowed to go to the gate as long as they don’t cross the boundary.”

Officials reported six arrests overall, mostly for misdemeanour trespassing on base property, an offence punishable with a $1,000 (£800) fine.