South Koreans have stopped their cars and huddled in underground shelters during the country's biggest-ever civilian evacuation drill.

Wednesday's 20-minute exercise was a response to soaring tensions after North Korea attacked a front-line South Korean island with artillery last month. Two civilians and two military personnel were killed.

The National Emergency Management Agency expected up to 12-million people to join the training.

During the drill, South Koreans were asked to evacuate to underground shelters after stopping work or parking their cars along streets.

A dozen South Korean jet fighters simulated air strikes, and a bullet train ran at a dramatically lower speed.

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