The tweet, posted on Friday on the Twitter account of the Defence Visual Information Distribution Service (DVIDS), took aim at UFO fans and curiosity seekers who poured into the Nevada desert this week, after an online campaign to “storm” the US military base long rumoured to house government secrets about extraterrestrial life and spaceships.

Alongside a photo of military men and women standing at attention in uniform in front of a B-2 stealth bomber, it read, “The last thing #Millennials will see if they attempt the #area51raid today.”

On Saturday, DVIDS said on Twitter that an employee of its DVIDSHub account posted a tweet that “in NO WAY supports the stance of the Department of Defence. It was inappropriate and we apologise for this mistake.”

In Nevada, any fears about a serious attempt to raid Area 51 appeared to have been unfounded.

About 150 people, some in alien garb, gathered near the base on Friday in a festive atmosphere with only a handful of arrests.

The US military has disowned previous social media posts that some people also criticised as threatening or insensitive.

On Dec 31, US Strategic Command, which oversees the country’s nuclear arsenal, apologised for a Twitter message that said it was ready if necessary to drop something “much, much bigger” than the New Year’s Eve ball in New York.

And last year the US Air Force apologised for a tweet that sought to find humour in killing Taliban militants in Afghanistan by invoking a viral Internet debate about whether an audio file says the words “Laurel” or “Yanny.”