SEOUL—The race to rescue possible survivors from a sunken South Korean passenger ferry intensified on Thursday, as rain, strong currents and low visibility hampered the search for nearly 300 people, including many missing 16- and 17-year-olds on a spring-break trip.

More than 500 professional divers from South Korea's coast guard and navy, along with 29 aircraft and 169 vessels, have been deployed to locate the victims, according to the government's anti-disaster agency.

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