G. Michael Harris, an experienced diver who has been to view the Titanic wreckage, fears the submersible that vanished on Sunday may have imploded thousands of feet underwater.
The prime minister was speaking ahead of this year s summit of the G7 group - which includes Canada, France, Germany, Italy, Japan, the United Kingdom and the United States.
Two US Army Black Hawk helicopters crashed during a training mission in Kentucky on Wednesday night causing “several casualties”, military officials said.
The Fort Campbell base has said the "status of the crew members are unknown" after the collision on Wednesday night. Kentucky governor Andy Beshear said "fatalities are expected".