The US Coast Guard was on Friday searching for a tour helicopter with seven people on board that went missing off the coast of Hawaii in "challenging" weather conditions.The chopper was on a tour of the coast of the northwestern island of Kauai. A boat and a helicopter were sent to scour the area when its owner raised the alarm around 40 minutes after it was due back on Thursday evening, the US Coast Guard said.There is an electronic locator on board but no signals have been received, it said.A pilot and six passengers were said to be in the helicopter - two of them reportedly minors."The weather conditions are challenging," said Petty Officer 1st Class Robert Cox according to local media. Visibility was low and there were blustery winds, he added."We have trained crews responding and on scene searching for any signs of the helicopter and those aboard."