96.11 points vanished from Seoul's benchmark KOSPI at the open, leaving it at 7,000.78 as investors locked in recent gains.
Wall Street set the negative tone overnight, with the S&P 500 down 1.21% and the Nasdaq off 2.15% as the Middle East war showed little sign of easing.
Donald Trump added to market anxiety by saying he is considering a "massive attack" on Iran beyond earlier strikes.
Brent crude futures climbed above $100 a barrel after Yemen's Houthis said they attacked ships in the Red Sea, deepening fears of a wider regional disruption.