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The hacking collective Anonymous has said it has been "hacking" and vandalising social networking profiles linked to North Korea.

The group has issued several warnings since the country's threats have intensified.

Uriminzokkiri, a news site, has been forced offline - while Twitter and Flickr accounts have been breached.

Anonymous also claimed to have accessed 15,000 usernames and passwords from a university database.

As part of action which the loosely organised collective has called "Operation Free Korea", the hackers have called for leader Kim Jong-un to step down, a democratic government to be put in place - and for North Koreans to get uncensored internet access.

Currently, only a select few in the country have access to the "internet" - which is more akin to a closed company intranet with only a select few websites that are government-run.

The country recently allowed foreigners to access mobile internet, but this service has since been shut off.

In a message posted online, members of Anonymous wrote: "To the citizens of North Korea we suggest to rise up and bring [this] oppressive government down!

"We are holding your back and your hand, while you take the journey to freedom, democracy and peace.

"You are not alone. Don't fear us, we are not terrorist, we are the good guys from the internet. AnonKorea and all the other Anons are here to set you free."

Urminzokkiri's Twitter feed started displaying messages reading "hacked" at around 0700 BST. The account's avatar was changed to a picture of two people dancing, with the words "Tango down".

On , other images, including a "wanted" poster mocking Kim Jong-un, were also posted.

Anonymous has posted what it said was a sample of the hacked information.

However, some have questioned the reliability of the details as some of the email addresses were in fact Chinese.

Also unreachable on Thursday was the website of Air Koryo, the country's airline, which launched its online booking site late last year.

Like the main Urminzokkiri homepage, it is suspected the Air Koryo site has been hit with a Distributed Denial of Service attacked (DDoS) - a technique which involves flooding a website with too much traffic for it to handle.

Although a highly secretive nation, North Korea puts considerable effort in to having a strong presence online.

Various YouTube accounts attached to the regime post news items and propaganda videos on a regular basis.