A solar observatory in the US state of New Mexico aimed at unlocking the sun's secrets has been shut indefinitely by the FBI over an undisclosed security issue, leaving it at the centre of its own mystery.

Key points:

• The observatory has been closed since staff were evacuated on September 6
• The FBI has referred all questions to the observatory's manager
• The facility is close to two military installations

The Sunspot Solar Observatory, near Holloman Air Force Base and White Sands Missile Range, sits atop the Sacramento mountains in southern New Mexico.

It has been closed since the FBI evacuated staff on September 6.

The facility's location, near military installations two hours' drive from the town of Roswell, has sparked speculation on social media.

Roswell is also known as the sight of a famous 1947 sighting of what witnesses claimed were UFOs, but the Air Force later said were top-secret high altitude weather balloons.

"Sounds like a case for Mulder & Scully," tweeted Ian Caverney, referring to the 1990s science fiction television series The X-Files.

"This is very abnormal," Everisto Gomez, a dispatch operator for the Otero County Sheriffs Office, said on Friday.

The FBI has not responded to requests for comment, referring questions to the observatory's manager, the Association of Universities for Research in Astronomy (AURA).

The group said in an email on Friday it would remain closed until further notice, citing an unspecified "ongoing security concern".

"[Aura] is addressing a security issue at the National Solar Observatory (NSO) facility at Sacramento Peak, New Mexico and has decided to temporarily vacate the facility as a precautionary measure until further notice," it said in a message on its website.

Its sunspot activity data is used nationwide. Sunspots are regions of reduced temperatures on the sun that appear as dark spots on the surface.

"Our people just locked up their offices and made their way to all their homes around the area" after the closure, Dr James McAteer, who runs the solar telescope at the observatory, said in an interview.