At least seven people were killed when a tour boat struck a cruise ship and sank into the Danube River in Budapest, Hungarian authorities said.

According to The Associated Press, the incident happened Wednesday evening as the boat returned from an outing organized by the Very Good Tour agency, which is based in South Korea.

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Officials said seven survivors, all South Koreans, were rescued and taken to a nearby hospital in stable condition, the AP reported. Twenty-one people are still missing, including 19 South Koreans and two Hungarians.

The small boat, called the Hableany, can hold up to 60 people. After the collision, it "turned on its side and sank in about seven seconds" near the Margit Bridge, the AP reported.

Rainy weather has complicated the rescue efforts, a police official told the AP. South Korean officials said they are headed to Hungary to help with the search.

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– The Associated Press contributed to this report.