WASHINGTON - The White House lifted a 40-year-old ban on taking photos during public tours of the executive mansion on Wednesday and invited visitors to share their shots on Twitter using #WhiteHouseTour.

First lady Michelle Obama announced the change in a video on Instagram.

"If you've been on a White House tour, you may have seen this sign," she said, holding up a sign reading "No Photos or Social Media allowed." "Well, not any more," Obama said as she tore up the sign, laughing.

The White House did not give a reason for the policy change but said some cameras and accessories such as so-called selfie sticks would still be prohibited.

Video cameras, cameras with detachable lenses, tablets, tripods, monopods will also remain banned. Phone cameras and compact still cameras with a lens no longer than 3 inches (7.6 cm) will be allowed, the White House said.

A relaxation of the camera rule coincided with a new security measure surrounding the executive mansion on Wednesday: a spiked top fence to thwart would-be intruders.

Sharp metal points will be bolted on top of the black iron fence as a temporary measure until authorities put up a more permanent structure next year. The changes were sparked by security breaches at the White House, including a September intrusion when a man scaled the fence and ran into the mansion.