Despite sharing a common language and ethnicity and centuries of history, North and South Korea have become radically different societies in recent decades.

Ruled with an iron grip by three generations of the Kim family, the North has turned itself into a nuclear power -- at the cost of international isolation -- while the South has embraced democracy and risen to become the world's 11th-largest economy.

The South's President Moon Jae-in regularly affirms unification as an eventual goal, but the picture in his country is far more nuanced, the survey by Seoul's Korea Institute for National Unification (KINU) showed.

A total of 65.9 percent of South Koreans saw unification as necessary, it said, down from 70.7 percent last year as inter-Korean engagement and nuclear talks between Pyongyang and Washington stall.

But when offered an alternative, 49.5 percent of Southerners favoured peaceful co-existence with the North with only 28.8 percent preferring unification, the biggest difference the survey has shown.

The differences are larger among younger people, with those in their 20s having spent their adult lives living with and sometimes threatened by a nuclear North.

If given a choice between unification and the economy, 70.5 percent put a higher priority on the economy, with just 8.3 percent backing unification.

A year ago the preferences were 60.7 and 12.8 respectively, but discontent over the South Korean economy is rising amid persistent unemployment and faltering growth.

The researchers carried out just over 1,000 face-to-face interviews last month, after North Korean leader Kim Jong Un and US President Donald Trump failed to reach a deal over denuclearisation and sanctions relief in Hanoi in February, and before North Korea last week carried out its first missile launches for more than a year.