The US Air Force has urged people not to invade Area 51 in search of aliens.Anyone attempting to do so will have to deal with highly developed defence measures, the military has said.A widespread internet joke – joined by more than a million people – suggests that people attempt to run into the Area 51 base on 20 September. If enough people do so, then at least some will be able to get through and storm the base, the Facebook group suggests.“They can’t stop all of us” it says. ”Let’s see them aliens.”Officials say they will not discuss the details of the security measures that are built to keep people out of the base. But they stressed that they are powerful and suggested soldiers are trained for this kind of situation.The range provides “flexible, realistic and multidimensional battlespace” for testing and “advanced training in support of US national interests”, it said.Nellis Air Force Base said in a statement that officials are aware of the Facebook posting and “any attempt to illegally access the area is highly discouraged”.After decades of government officials refusing to acknowledge Area 51, the CIA released declassified documents in 2013 referring to the 8,000-square mile installation by name and locating it on a map near the dry Groom Lake bed.The base has been a testing ground for a host of top-secret aircraft, including the U-2 in the 1950s and later the B-2 stealth bomber.Secrecy surrounding the site has fuelled conspiracy theories among UFO enthusiasts and sprouted a small, alien-themed tourist industry in surrounding desert communities, including cafes, a motel and even a brothel.Additional reporting by agencies