Videos posted on YouTube show what happens when curious visitors reach the warning signs of the US Air Force secret facility deep in the Nevada desert.

In one, two men stumble across a secret rear entrance to Area 51, which they claim had warning signs saying use of deadly force is authorised.

They also claim that perched on top of a rocky hill was a sniper aiming a gun at them.

One of them said: "It looks like a sniper man. That is where the cammo guys are watching this from.

"And it says use of deadly force authorised."

The second man said: "What is that? If they're not holding some kind of incredible items why do they have what looks like guns.

"I didn't think it was as extreme as they say it is, we have been to every other gate and this is the only one that says use of deadly force.

"What do you have in there that is so secret, you going to have a sniper out there perched up on a hill?"