In a move to limit access to the internet, North Korea allows mobile phone SIM cards used by tourists to be active only for the duration of their visit, tourism sources say.
Unlike North Koreans, foreigners visiting the isolated country can browse social media sites such as Facebook and Twitter freely, using the Koryolink domestic network.
Under a change made in July, North Korea would deactivate the card when a visitor left, ensuring that it could not be left for use by a resident, the sources said. It could then be reactivated when a visitor returned to the country.
"This basically means in practical terms that if someone leaves the country they can't simply leave their phone with a local friend and have them use the internet," said one source.
The move could be linked to a broader crackdown on the exchange of information in North Korea, and according to the source appeared to have been government-led.
More than 2.5 million North Koreans use the Koryolink network to make calls and browse an internal, heavily monitored domestic internet. Foreigners can also use the network, but on a separate cell network that connects to the global internet. It was not clear if the new rule applied to contracts held by long-term residents or foreign diplomats.
Koryolink is a joint venture with Egypt's Orascom Telecom.
Information in North Korea is tightly controlled but storage devices such as USB sticks or micro SD cards have become popular in recent years as they allow people to discretely share uncensored information such as videos, games, music and e-books.
SIM cards used in phones to access mobile networks are also easily concealed.
Sokeel Park of LiNK, an NGO that works with North Korean defectors, said Pyongyang had stepped-up control of information flows, resulting in a crackdown on illegal foreign media and smuggled Chinese cell phones that are often used to make international calls from areas in North Korea within range of Chinese cell towers along the border with China.