SEOUL: In a move that makes it harder for North Koreans to gain illicit access to the global Internet, North Korea now only allows mobile phone SIM cards used by tourists to be active for the duration of their visit, tourism sources said.

Unlike North Koreans, foreigners visiting the isolated country can freely browse social media sites such as Facebook and Twitter using the Koryolink domestic network.

Under a change made in July, North Korea deactivates the card when a visitor leaves, ensuring that it can not be left for use by a resident, the sources said. It can be reactivated when a visitor returns 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, who declined to be identified due to the sensitivity of discussing such issues when working in North Korea.

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 use the network too - but on a separate cell network that connects to the regular outside 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 repressive North Korea is tightly controlled but small storage devices like USB sticks or micro SD cards have become popular in recent years for discretely sharing uncensored information such as videos, games, music and ebooks.

SIM cards used in phones to access mobile networks are also easily concealed, and experts say the new policy could be linked to a wider crackdown.

Sokeel Park of LiNK, an NGO that works with North Korean defectors, said Pyongyang has stepped-up control of information flows under Kim Jong Un, who came to power in late 2011 when his father Kim Jong Il died.

Park was referring to a widespread 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.

“It would make sense to close a loophole that might have seen some foreigners lend their SIM cards to North Koreans while they were away, since international phone calls and 3G access to the global internet are a big breach of their information blockade with the outside world,” said Park.