The move to "Pyongyang Time" coincided with the 70th anniversary of the Korean peninsula's 1945 liberation from Japanese colonial rule, and means the two Koreas now operate in different time zones.

The South has criticised the change for placing a fresh obstacle in the path of eventual re-unification, while Pyongyang has mocked Seoul for remaining under the colonial yoke and sticking to the same time zone as Tokyo.

Standard time in pre-colonial Korea had run at GMT+8:30 but was changed to Japan standard time in the early years of the 1910-45 Japanese occupation.

Midnight came twice for North Koreans on Friday night as they set their clocks back half-an-hour, and state television showed men in traditional Korean costume ringing a giant ceremonial bell in celebration.

At the same time, factories, trains and ships across the country sounded their whistles and sirens.

The use of Pyongyang Time is "aimed at erasing all traces of Japanese colonial rule," the state TV announcer said.

Seoul's Unification Ministry, which deals with cross-border affairs, warned earlier this week that a different time zone between North and South Korea posed a number of possible challenges, including for operations at the jointly-run Kaesong industrial complex that lies just inside North Korea.

South Korean President Park Geun-Hye criticised the move as "regrettable", saying it would deepen divisions between the two Koreas.

The North responded by calling her a Japanese "sycophant".

South Korea had similarly turned its clocks back in 1954 but reverted to Japan standard time in 1961 after President Park's father, Park Chung-Hee, came to power in a military coup.

Park's rationale was partly that the two major US allies in the region -- South Korea and Japan -- should be grouped in the same time zone to facilitate operational planning.(+++)

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