Approximately 630 billion won ($457.3 million) in trades by South Korean investors were canceled following a recent suspension in after-hours trading in the US largely due to a surge in order volume, Korean authorities said Thursday.

According to the Financial Supervisory Service, the cancellations affected around 90,000 accounts at 19 local securities firms that offer daytime trading of US stocks to Korean investors.

On Monday afternoon Korean time, US overnight trading platform Blue Ocean ATS shut down its matching engine due to an unexpected surge in trade volume. This system, which facilitates after-hours US stock trading for overseas investors, led to the cancellation of all orders placed between 2:45 and 4:06 p.m. Consequently, all gains and losses from the orders were also nullified.

“We have determined that Blue Ocean's trading system, which primarily serves Asian investors, malfunctioned due to exceeding its processing limits amid a surge in order volume,” an FSS official said.

Amid escalating concerns about a US economic downturn, global stock market volatility has intensified over the past week. Asian markets, including in Korea, plummeted sharply Monday, fueling fears of potential declines in the US market and leading to a surge in orders by Asian investors in the US overnight.

Brokerage firms in the US, including major players like Robinhood, which uses Blue Ocean ATS for extended trading hours, were forced to halt their around-the-clock service.

As the issue originated from a system error on the overseas trading platform, the FSS indicated that local brokerages are unlikely to be held responsible and stated authorities would support voluntary resolutions between the companies and investors. As of Wednesday afternoon, the FSS had received over 100 complaints related to the incident.

Since the shutdown on Monday, Blue Ocean ATS has remained largely suspended. Trading resumed for only 29 exchange-traded funds symbols, including the Nasdaq 100-tracking QQQ, TQQQ and S&P 500-tracking SPY, as of Wednesday.

Meanwhile, local brokerages announced Thursday that restrictions on the after-hours trading of US stocks will continue until Aug. 15, based on an announcement by Blue Ocean ATS. The platform stated it will resume trading of the full National Market System symbol list starting from the Aug. 16 session.