Image copyright AP Image caption Bells rang out at midnight in Pyongyang as clocks were put back half an hour

North Korea has switched to a new time zone, turning its clocks back by 30 minutes to mark its liberation from the Japanese at the end of WW2.

Bells rang out in the capital Pyongyang at midnight on Friday as the new time zone came into effect.

North Korea announced the move earlier this week, surprising South Korea whose president warned it ran counter to efforts to foster co-operation.

The North had been nine hours ahead of GMT - like South Korea and Japan.

But before being colonised by Japan in 1910, the entire Korean peninsula - then one country - was 8.5 hours ahead of GMT.

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Image copyright AP Image caption The bell-ringing ceremony was watched by North Koreans

North Korea's chief astronomer Jong Sok said changing time zones made a lot of sense - and was appropriate as the country marked 70 years since it was liberated from Japanese rule.

"With the time standard that we have used up until now, the time when the sun is at its highest position is not correctly noon," he told the Associated Press.

"I think it is the lawful right of a sovereign state that our republic - to mark the 70th anniversary of our liberation and the 70th anniversary of the defeat of Japanese imperialism - has announced our time as Pyongyang Time, the same as our ancestors used and which was robbed from us by the Japanese imperialists."

State news agency KCNA said earlier this week "wicked Japanese imperialists" had "deprived Korea of even its standard time" by changing the clocks during the occupation.

South Korean president Park Geun-hye said on Monday Pyongyang's "unilateral" decision, taken without any consultation with Seoul, was "highly regrettable" and threatened a "further deepening of disparity between the two Koreas".