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A court hearing is underway in Hungary to decide whether the captain of a river cruise ship should be arrested after his vessel collided on the Danube River with a sightseeing boat, killing seven South Korean tourists.

Prosecutors are requesting that the 64-year-old Ukrainian captain of the Viking Sigyn be arrested. The captain has denied responsibility in Wednesday's collision in central Budapest.

The smaller sightseeing boat was carrying 33 South Koreans and two Hungarian crew when it collided with the Viking vessel. Seven people were rescued and 21 are still missing.

Efforts to salvage the 27-meter (88.5-foot-long) tour boat have been hampered by the river's fast flow and limited visibility under water.

Specialists from South Korea and Austria are assisting in the salvage efforts being coordinated by Hungary's Counter-Terrorism Center.