North Korea blocks South workers from Kaesong zone

North Korea has stopped South Koreans from crossing the border to work at the jointly run Kaesong industrial zone, for the first time since 2009.

Seoul said about 800 South Koreans who had stayed overnight at the complex were being allowed to return.

Kaesong is a crucial revenue source for the North, which has not indicated how long the entry ban will last.

Pyongyang has threatened the South and the US in recent weeks, and has vowed to restart a mothballed nuclear plant.

The border into Kaesong is the last functioning crossing between the two Koreas, and the complex is the last significant symbol of co-operation.

The South's Unification Ministry spokesman Kim Hyung-seok told reporters he wanted the ban to be lifted immediately.

"Ensuring the safety of our citizens is our top priority and the South Korean government will take necessary measures based on this principle," he said.