At least 11 people have died after an amphibious "duck boat" carrying tourists capsized in stormy weather in the US state of Missouri, police said.Stone County Sheriff Doug Rader said five people are unaccounted for and seven others were injured in Thursday night's incident on Table Rock Lake near Branson.The boat was carrying 31 people.Rescue operations were continuing on Friday, CNN reported.A video footage shot by a witness on shore showed two duck boats rocking and tilting to the side as the lake's ripples turned into massive waves. One of the boats made it to shore but the other was driven back by the wind and gradually swamped.According to weather reports, sustained winds of 65 kmph with gusts of up to 100 kmph had struck the area at the time of the accident. Rader said one of his deputies was on board at the time and had helped to rescue some of the passengers.He said there were life jackets aboard the boat, but he doesn't know whether people were wearing them.A spokeswoman for Ripley Entertainment, the parent company of Ride the Ducks Branson, Suzanne Smagala-Potts said the company recently acquired the vessel involved in the incident. Other boats on the lake had returned to the dock safely.The amphibious boat travels on both land and water, and is popular among tourists in major cities. The boats' history dates back to World War II, when such boats were a common sight due to their versatility.