The DMZ dividing the Korean peninsula is one of the most heavily fortified places in the world, and almost all defectors to the South go to China first -- where they still risk being repatriated if caught -- and then on to a third country before travelling to the South.

In the six months to June, 593 Northerners entered South Korea, down 20.8 percent from the same period in 2016, statistics compiled by Seoul's Unification Ministry showed.

As usual most -- 85 percent -- were women. North Korean men who try to leave are likely to be rapidly identified as absent by their work units.

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Pyongyang's "tightened grip on the population and strengthened border controls add to the risks for potential defectors to take the plunge", a ministry official told AFP.

The Seoul-financed Korea Institute for National Unification said in a report that since late 2015, the North has been bolstering border controls and installing high-tension electric fencing along the Tumen River that forms the border with China.

A total of 30,805 North Koreans have fled to the South, many of them leaving during the famine years of the 1990s.

Arrivals peaked in 2009, but numbers have fallen more recently, with leader Kim Jong-Un reportedly ordering crackdowns on defectors and tightened border controls after inheriting power from his father in 2011.